Surface coupled systems

ABSTRACT

A system includes a surface coupled edge emitting laser that includes a core waveguide, a fan out region optically coupled to the core waveguide in a same layer of the surface coupled edge emitting laser as the core waveguide; and a first surface grating formed in the fan out region; and a photonic integrated circuit (PIC) that includes an optical waveguide and a second surface grating formed in an upper layer of the PIC, wherein the second surface grating is in optical alignment with the first surface grating.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims benefit of and priority to U.S.Provisional App. No. 62/268,907 filed Dec. 17, 2015 and to U.S.Provisional App. No. 62/379,569 filed Aug. 25, 2016 which areincorporated herein by reference.

FIELD

The embodiments discussed herein are related to surface coupled systems.

BACKGROUND

Unless otherwise indicated herein, the materials described herein arenot prior art to the claims in the present application and are notadmitted to be prior art by inclusion in this section.

Coupling light from single mode edge emitting lasers to silicon (Si)photonics is costly, as it generally requires two lenses and a largeisolator block. In systems that include such lasers and Silicon (Si)photonics, alignment tolerances may be less than 0.5 micrometers (μm).Such low alignment tolerances typically require active alignment to bemet.

The subject matter claimed herein is not limited to implementations thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one example technology area where some implementationsdescribed herein may be practiced.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential characteristics of the claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter.

Some example embodiments described herein generally relate surfacecoupled systems.

A system may include a surface coupled edge emitting laser that includesa core waveguide, a fan out region optically coupled to the corewaveguide in a same layer of the surface coupled edge emitting laser asthe core waveguide; and a first surface grating formed in the fan outregion; and a photonic integrated circuit (PIC) that includes an opticalwaveguide and a second surface grating formed in an upper layer of thePIC, where the second surface grating is in optical alignment with thefirst surface grating.

Another system may include a surface coupled edge emitting laser thatincludes a first waveguide and a first diffraction grating opticallycoupled to the first waveguide; and a PIC that includes a secondwaveguide and a second diffraction grating optically coupled to thesecond waveguide, where the first waveguide of the surface coupled edgeemitting laser includes a core with a core index of refraction, a topcladding with a top cladding index of refraction, and a substrate as abottom cladding with a bottom cladding index of refraction; where thefirst diffraction grating includes grating teeth formed on the core ofthe first waveguide, the grating teeth each having a total height, aheight above the core of the first waveguide, a period, and a dutycycle; and where the core index of refraction is greater than a firstthreshold value so that an effective index of the first diffractiongrating is sufficiently higher than the bottom cladding index to avoidleakage of a diffracted optical mode into the substrate.

Yet another system may include a surface coupled edge emitting laserthat includes a first waveguide and a first diffraction gratingoptically coupled to the first waveguide; and a PIC that includes asecond waveguide and a second diffraction grating optically coupled tothe second waveguide, where the first waveguide of the surface couplededge emitting laser includes a core with a core index of refraction, atop cladding with a top cladding index of refraction, and a substrate asa bottom cladding with a bottom cladding index of refraction; where thefirst diffraction grating includes alternating grating teeth and topcladding teeth formed above the core of the first waveguide; and wherean effective index of the first diffraction grating depends on at leastthe core index of refraction and the top cladding index of refractionand is at least 6% higher than the bottom cladding index of refraction.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 illustrates an example surface coupled system;

FIGS. 2A and 2B illustrate an example surface coupled edge emittinglaser that may be implemented in a surface coupled system;

FIG. 2C illustrates another example surface coupled edge emitting laserthat may be implemented in a surface coupled system;

FIGS. 3A and 3B illustrate another example surface coupled system;

FIG. 4 illustrates three different example optical isolatorconfigurations;

FIG. 5 illustrates a graphical representation of a simulation ofcoupling efficiency as a function of a gap distance z for multiple spotsizes;

FIG. 6 illustrates a passive section of an example surface coupled edgeemitting laser;

FIG. 7 illustrates a passive section of another example surface couplededge emitting laser;

FIGS. 8A and 8B each include a graphical representation of a simulationof light propagation through a passive section;

FIG. 9A illustrates a graphical representation of an example far fieldprofile as a function of diffraction angle for a passive section;

FIG. 9B illustrates various graphical representations of simulations fordiffracted light;

FIG. 10 illustrates a graphical representation of a simulation ofdiffraction efficiency loss as a function of number N of grating periodsof a first surface grating in a passive section with a top mirror;

FIG. 11 illustrates another passive section of a laser;

FIG. 12 illustrates a side cross-sectional view, a shallow ridgeend-oriented cross-sectional view, a deep ridge end-orientedcross-sectional view, and an overhead view of another example surfacecoupled edge emitting laser;

FIG. 13 illustrates various graphical representations of electric fieldof grating output as a function of location along a length of a firstsurface grating and a far field profile as a function of diffractionangle of the first surface grating;

FIG. 14A illustrates a side cross-sectional view of a surface couplededge emitting laser;

FIG. 14B illustrates another example surface coupled system;

FIG. 15 illustrates an overhead view of a core waveguide, a fan outregion, and a first surface grating;

FIG. 16 illustrates a graphical representation of simulated lightintensity within the core waveguide, the fan out region, and the firstsurface grating of FIG. 15;

FIG. 17 illustrates another example surface coupled system;

FIG. 18A illustrates another example of a surface coupled edge emittinglaser;

FIGS. 18B and 18C illustrate the laser of FIG. 18A at various processingsteps;

FIGS. 19A and 19B include an overhead view and a side view of an exampleSi photonic communication module;

FIG. 20A illustrates another example Si photonic communication module;

FIG. 20B illustrates another example Si photonic communication module;

FIG. 21 illustrates another example surface coupled system;

FIG. 22 illustrates various views of an example surface coupled edgeemitting laser that may be implemented in the surface coupled system ofFIG. 21;

FIG. 23 illustrates various views of another example surface couplededge emitting laser that may be implemented in the surface coupledsystem of FIG. 21;

FIG. 24 illustrates another example surface coupled system;

FIG. 25A illustrates an example Si PIC;

FIG. 25B illustrates another example Si PIC;

FIG. 26 illustrates a side cross-sectional view of a SiN LASG in a PICwith a mirror;

FIG. 27 illustrates another example surface coupled system;

FIG. 28 illustrates an example focusing surface grating that may beimplemented in one or both of the first and second surface gratings orother LASGs described herein;

FIG. 29 depicts an example concept to increase directionality of asurface grating that may be implemented in one or both of the first andsecond surface gratings described herein;

FIG. 30A illustrates another example surface coupled system;

FIG. 30B illustrates an example implementation of the surface coupledsystem of FIG. 30A;

FIG. 31 illustrates another example surface coupled system; and

FIG. 32 illustrates another example surface coupled system.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

Some embodiments described herein remove the need for lenses in systemssuch as those described above as generally requiring two lenses and alarge isolator block, which may reduce part count and cost in suchsystems, and significantly simplify packaging processes in such systems.An isolator may be used in such systems. The absence of lenses in suchsystems may significantly reduce the size and cost of the isolator andmay significantly increase alignment tolerances. For example, thealignment tolerance may be increased by a factor of 10 or even 50 ormore from about 0.1 μm which has to be done by active feed-backalignment, which requires turning on the laser during alignment, toabout 1-2 μm or even 5-8 μm or more achieved in a passive alignmentpick-and place machine; i.e. without having to turn on the laser.Alternatively or additionally, embodiments described herein may enablewafer level testing of lasers.

According to some embodiments, a surface coupled system including afirst surface grating (or first diffraction grating or transmit grating)and a second surface grating (or second diffraction grating or receivegrating) is provided to couple light from an edge emitting laser to aPIC, such as a Si PIC. In some embodiments, the first and second surfacegratings may each include a small index contrast long surface grating.In general, a small index contrast long surface grating may include asurface grating with an index contrast less than about 1-1.5 and alength greater than 10 μm. In other embodiments, the first and secondsurface gratings may each include a large area surface grating (LASG)with a length greater than about 10 μm and with or without small indexcontrast.

The edge emitting laser may include an indium phosphide (InP) laser orother suitable edge emitting laser. The InP laser may include an inputpassive waveguide that expands in a fan out region to the first surfacegrating. The first surface grating may be configured to generate arelatively large optical mode spot size of about 8-40 μm for an opticalbeam diffracted by the first surface grating.

The second surface grating may be formed in the Si PIC. The secondsurface grating may be configured to receive the optical beam diffractedby the first surface grating and to redirect the optical beam into awaveguide of the Si PIC.

Embodiments described herein additionally include aspects of the firstdiffraction grating. In an example embodiment, a surface coupled systemmay include a surface coupled edge emitting laser and a PIC. The surfacecoupled edge emitting laser may include a first waveguide and a firstdiffraction grating optically coupled to the first waveguide. The PICmay include a second waveguide and a second diffraction gratingoptically coupled to the second waveguide. The first waveguide of thesurface coupled edge emitting laser may include a core with a core indexof refraction, a top cladding with a top cladding index of refraction,and a substrate as a bottom cladding with a bottom cladding index ofrefraction. The first diffraction grating may include grating teethformed on the core of the first waveguide, the grating teeth may eachhave a total height, a height above the core of the first waveguide, aperiod, and a duty cycle. The core index of refraction may be greaterthan a first threshold value so that an effective index of the firstdiffraction grating is sufficiently higher than the bottom claddingindex to avoid leakage of a diffracted optical mode into the substrate.

The core index of refraction which in some embodiments may be the sameas a grating tooth index of refraction of the grating teeth may begreater than or equal to 3.4, such as in a range from 3.4 to 3.44, orequal to 3.42. Various other parameters of the first diffraction gratingaccording to one or more example embodiments are described below.

In another example embodiment, a surface coupled system may include asurface coupled edge emitting laser and a PIC. The surface coupled edgeemitting laser may include a first waveguide and a first diffractiongrating optically coupled to the first waveguide. The PIC may include asecond waveguide and a second diffraction grating optically coupled tothe second waveguide. The first waveguide of the surface coupled edgeemitting laser may include a core with a core index of refraction, a topcladding with a top cladding index of refraction, and a substrate as abottom cladding with a bottom cladding index of refraction. The firstdiffraction grating may include alternating grating teeth and topcladding teeth formed above the core of the first waveguide. Aneffective index of the first diffraction grating may depend on at leastthe core index of refraction and the top cladding index of refractionand may be at least 6% higher than the bottom cladding index ofrefraction.

Reference will now be made to the drawings to describe various aspectsof example embodiments of the invention. It is to be understood that thedrawings are diagrammatic and schematic representations of such exampleembodiments, and are not limiting of the present invention, nor are theynecessarily drawn to scale.

FIG. 1 illustrates an example surface coupled system 100, arranged inaccordance with at least one embodiment described herein. The surfacecoupled system 100 may include a surface coupled edge emitting laser(hereinafter “laser”) 102 and a Si PIC 104. In at least one embodiment,the laser 102 may include an InP laser.

Additionally, the laser 102 may include a first surface grating 106 andthe Si PIC 104 may include a second surface grating 108. The firstsurface grating 106 may be optically coupled to an active section 112 ofthe laser 102 through a core waveguide. The core waveguide may beoptically coupled to receive light emitted by a gain medium (notillustrated) of the active section 112 of the laser 102. In someembodiments, a fan out region may be provided between the core waveguideand the first surface grating 106 and/or may include the core waveguide.The fan out region may be formed from a same medium and layer as thecore waveguide such that the fan out region may generally be anextension of the core waveguide. Additionally, the fan out region mayinclude grating lines such that the fan out region may generally be anextension of the first surface grating 106.

The light emitted from the active section 112 of the laser 102 maytravel through the core waveguide to the fan out region, where a mode ofthe light may be expanded laterally (e.g., generally in and out of thepage in FIG. 1). The first surface grating 106 may diffract the lightwith the laterally expanded mode generally downward as diffracted light110. The diffracted light 110 may be diffracted toward the secondsurface grating 108 of the Si PIC 104. The mode of the diffracted light110 may be expanded to a 8-40 μm spot size (lateral measurement) withinthe fan out region while simultaneously being expanded along thedirection of the active section 112 by the first surface grating 106.One potential benefit of this method of expanding diffracted light maybe that the spot size may be much larger than the 2 to 4 μm spot sizethat can be achieved with standard spot size converters.

The diffracted light 110 may be received by the second surface grating108. The diffracted light 110 may be redirected by the second surfacegrating 108 into a waveguide (not illustrated) of the Si PIC 104. Oneexample of the waveguide may be a Si waveguide. Although notillustrated, an optical isolator may be provided between the firstsurface grating 106 and the second surface grating 108 to reduce backreflection. The optical isolator may be attached to the Si PIC 104and/or the laser 102.

In other laser-PIC systems, one potential issue may be that two lensesand a large optical isolator block may be used to couple light from alaser to a Si waveguide of a PIC due to the relatively small spot sizein such systems. These other laser-PIC systems and other similar systemsmay have an alignment tolerance of the laser relative to the PIC of 0.1μm. Another potential issue of these other laser-PIC systems and othersimilar systems is that alignment is generally actively performed (e.g.,performed with the laser on).

In comparison, the alignment tolerance between the laser 102 and Si PIC104, in accordance with systems and designs discussed herein, may be 1-2μm or more. Such an alignment tolerance may allow alignment to beperformed passively (e.g., laser off during alignment).

Surface gratings, such as the first surface grating 106 and the secondsurface grating 108 discussed herein, may include a periodic structure.The period structure may have an index of refraction that periodicallyalternates by providing repeated periodic regions of alternatingmaterials. The periodic regions may be called corrugations or teeth. Thecorrugations or teeth of the first surface grating 106 and the secondsurface grating 108 may be partially etched to improve directionality ofthe diffracted light 110.

The periodic regions of alternating materials may have a region with afirst material with a first index of refraction and another region witha second material with a second index of refraction. A differencebetween the first index of refraction and the second index of refractionmay be referred to as index contrast of the surface grating. A length ofthe periodic structure in a direction of light propagation may bereferred to as a length of the surface grating. A depth of the surfacegrating or of a corrugation included in the periodic structure, wheredepth is measured in a direction that is both orthogonal to the lightpropagation direction and orthogonal to the lateral expansion direction,may be referred to as Kappa, or K.

Gratings formed in InP are typically used as distributed feedbackreflectors (DBR) to form a mirror in a laser cavity. In theseapplications the grating reflects the light, which is incident from awaveguide in InP, directly back 180 degrees relative to the input and inthe same plane as the input, e.g., back reflection. The index contrastof the grating is between indium gallium arsenic phosphide (InGaAsP) andInP which have slightly different refractive indices depending on exactcomposition of InGaAsP. As an example of a DBR, the InP grating may havean index contrast of about 0.01 with a Kappa of 100-200 cm-1 and alength of about 100 μm. An example InP grating with these configurationsmay achieve about 92% coupling efficiency. The bandwidth of an InPsurface grating may be about 4 nanometers (nm) if the InP surfacegrating is centered on a nominal center wavelength of the laser.According to at least one embodiment described herein, however, InPgratings may be designed to diffract the incoming light out of the planeof incidence and so is a diffraction grating that may be referred to asa surface grating.

In at least one embodiment, an InP surface grating may have an indexcontrast of about 2 using deep etched InGaAsP or InP surface gratingswith a silicon dioxide (SiO2) or other dielectric top cladding and theInP surface grating may also have a length of about 50 μm to achieve alow coupling loss of −0.5 dB. Coupling loss may be a ratio of powerdirected out of a plane of the grating into a narrow angle in a farfield relative to a total power at an input of a waveguide.

The length of an example InP surface grating may be longer than thelength of an example Si/SiO₂ surface grating because the effective indexcontrast of an example InP surface grating, defined as the overlapintegral of the optical mode with the grating index profile, may berelatively smaller than the effective index contrast of an example Sisurface grating. An example InP surface grating may be designed with ahigh coupling efficiency using a bandwidth as high as 80 nm in someembodiments.

An example of the second surface grating 108 may include a Si surfacegrating. An example Si surface grating may have an index contrast ofabout 2 and a length of 10-20 μm. An example Si surface grating withthese configurations may be designed for far field emission into about a10 μm fiber mode over about a 30 nm bandwidth. An example Si surfacegrating may be designed to match a mode profile generated by an exampleInP surface grating. Alternatively, the Si PIC 104, as discussed herein,may include a silicon nitride (SiN) surface grating implemented as thesecond surface grating 108. A SiN surface grating may have a smallerindex contrast than a Si surface grating. As such, a mode profile for aSiN surface grating may better match a mode profile of an example InPsurface grating.

A coupling efficiency of an example Si surface grating with an indexcontrast of roughly 2 and a length between 10-20 μm may be between 0.8to 2 decibels (dB). An example Si surface grating with a smaller indexcontrast and/or a longer length may have a better coupling efficiencythan 0.8 to 2 dB. In some embodiments, the length of an example Sisurface grating may be extended to match or substantially match thelength of an example InP surface grating.

For surface gratings, such as the first surface grating 106 and secondsurface grating 108, as discussed above and herein, maximal coupling, orresonance, may occur at a resonance wavelength as defined in equation 1:

$\begin{matrix}{{{{\frac{2\;\pi}{\lambda_{0}}n_{s}\sin\;\theta} + \frac{2\;\pi}{\Lambda}} = {\frac{2\;\pi}{\lambda_{0}}n_{w}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$In equation 1, λ₀ may indicate the resonance wavelength, n_(s) mayindicate an index of refraction of a cladding of a surface grating, θmay be a coupling angle, Λ may be a grating period of a surface grating,and n_(w) may be an effective index of a surface grating. The effectiveindex n_(w) of the surface grating may depend on, e.g., a depth of anetch, a refractive index of a core, and a refractive index of adielectric, where the core and dielectric make up the periodic regionsof the periodic structure of the surface grating. For a fixed gratingperiod and coupling angle, the coupling efficiency may degrade as awavelength moves away from a resonance wavelength because a resonancecondition of Equation 1 is no longer satisfied.

Equation 2 introduces a wave vector mismatch quantity, as a function ofwavelength:

$\begin{matrix}{{{\Delta\;{\beta(\lambda)}} = {{\frac{2\;\pi}{\lambda}n_{w}} - \left( {{\frac{2\;\pi}{\lambda}n_{s}\sin\;\theta} + \frac{2\;\pi}{\Lambda}} \right)}},} & {{Equation}\mspace{14mu} 2}\end{matrix}$In equation 2, Δβ is the wave vector mismatch as a function ofwavelength λ and n_(s), n_(w), and Λ are as described with respect toEquation 2. At resonance, Δβ(λ) may equal to 0.

FIGS. 2A and 2B illustrate an example surface coupled edge emittinglaser (hereinafter “laser”) 202A that may be implemented in a surfacecoupled system, such as the surface coupled system 100 discussed inrelation to FIG. 1. FIG. 2A includes a bottom view and FIG. 2B includesa bottom perspective view of the laser 202A. FIG. 2C illustrates anotherexample surface coupled edge emitting laser (hereinafter “laser”) 202Bthat may be implemented in a surface coupled system, such as the surfacecoupled system 100 discussed in relation to FIG. 1. Each of the lasers202A and 202B may include or correspond to the laser 102 of FIG. 1.

Referring first to FIGS. 2A and 2B, the laser 202A may include a gainmedium 216, a first distributed Bragg reflector (DBR) 214A, and a secondDBR 214B. The first and second DBRs 214A-B together with the gain medium216 may form a laser cavity 212 such that the laser 202A in the exampleof FIGS. 2A and 2B may include a DBR laser. Alternatively oradditionally, and as illustrated in FIG. 2C, the laser 202B may includea distributed feedback (DFB) laser in which a grating 220 and gainmedium 222 overlap in the laser cavity. In other embodiments, a DFB typegain region and one or more passive DBR regions may both be present toprovide feedback in a configuration which may be termed a DistributedReflector (DR) laser, and which may be used for high speed laserapplications. Each of the lasers 202A, 202B may include a first surfacegrating 206 optically coupled to the corresponding laser cavity (e.g.,212 in FIGS. 2A and 2B). The first surface grating may be similar oridentical to the first surface grating 106 discussed in relation toFIG. 1. A fan out region of the first surface grating 206 may includegrating lines such that the first surface grating 206 and the fan outregion partially or completely overlap.

In FIGS. 2A and 2B, a reflectance of the second DBR 214B may be about 98percent and a reflectance of the first DBR 214A may be about 30 percent.In other embodiments, the first and second DBRs 214A-B may have otherreflectance values.

The laser 202A may generally emit light 218 through the first DBR 214Atoward the first surface grating 206. The emitted light 218 may interactwith the first surface grating 206 to be diffracted by the first surfacegrating 206 as diffracted light 210.

In FIG. 2C, the laser 202B implemented as a DFB laser may generally emitlight 224 through a front of the DFB laser toward the first surfacegrating 206. The light 224 may interact with the first surface grating206 to be diffracted by the first surface grating 206 as diffractedlight 226.

The laser 202A and/or 202B may be hermetically sealed by a passivationlayer formed by SiN or silicon oxide (SiO_(x)) deposition on the laser202A or 202B. In more detail, one or more layers of SiN and/or SiO_(x)may be deposited over the laser 202A or 202B to hermetically seal thelaser 202A or 202B.

FIGS. 3A and 3B illustrate another example surface coupled system 300,arranged in accordance with at least one embodiment described herein.The surface coupled system 300 may include a surface coupled edgeemitting laser (hereinafter “laser”) 302 and a Si PIC 304. The laser 302may be the same or similar to the laser 102, 202A, and/or 202B discussedabove in relation to FIGS. 1 and 2A-2C. The Si PIC 304 may be the sameor similar to the Si PIC 104 discussed above in relation to FIG. 1.

The laser 302 may include a first surface grating 306. The first surfacegrating 306 may be the same or similar to the first surface gratings 106and 206 discussed above in relation to FIGS. 1 and 2A-2C. The Si PIC 304may include a second surface grating 308. The second surface grating 308may be the same or similar to the second surface grating 108 discussedabove in relation to FIG. 1. The first surface grating 306 and thesecond surface grating 308 may alternatively or additionally eachinclude a LASG. Alternatively or additionally, the first surface grating306 may be referred to as a transmitter grating or a large area surfacelaser coupler, while the second surface grating 308 may be referred toas a receiver grating or a large area surface Si coupler. The surfacecoupled system 300 may also include an optical isolator 320 disposedbetween the laser 302 and the Si PIC 304.

The laser 302 may be configured to expand an optical mode of lightemitted from a laser cavity 312 of the laser 302. The optical mode oflight emitted from the laser cavity 312 may be expanded to a 8-40 μmspot size, or to a 20-40 μm spot size, or other relatively large spotsize. As such, an alignment tolerance of the laser 302 relative to theSi PIC 304 may be around +/−5 μm, which may be similar to alignmenttolerance of other lasers, such as a multimode (MM) vertical cavitysurface emitting laser (VCSEL). For such a relatively large spot size,diffraction may be negligible for the optical isolator 320. As such, insome embodiments, the optical isolator 320 may be around 600 μm thickincluding a garnet with an input polarizer on an upper surface of theoptical isolator 320. More generally, the optical isolator 320 may havea physical thickness in a range from 300 μm to 800 μm. The upper surfaceof the optical isolator 320 may be a surface that is coupled to thelaser 302.

The optical isolator 320 may include an output polarizer on a lowersurface of the optical isolator 320. The lower surface of the opticalisolator 320 may be a surface that is coupled to the Si PIC 304. Each ofthe input and output polarizers may include polarizers marketed byCORNING under the trade name POLARCOR, or other suitable polarizers. Theupper surface area or lower surface area of the optical isolator 320 maybe 100-200 μm² or less, compared to a surface area of at least 400-500μm² for optical isolators in other laser-PIC systems where the opticalisolator is positioned between two lenses. A smaller surface area of theoptical isolator 320 may reduce the cost of the optical isolator 320 insome embodiments compared to other laser-PIC systems.

A benefit of LASGs described herein can be seen by consideringdivergence of an optical beam from a laser, as illustrated in FIG. 3B.With reference to FIG. 3B, a spot size diffracting out of the laser 302may be determined in accordance with equation 3:

$\begin{matrix}{{{{w(z)} = {w_{0}\sqrt{1 + \left( \frac{z}{z_{R}} \right)^{2}}}},{where}}{z_{R} = \frac{\pi\;{nw}_{0}^{2}}{\lambda}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$In equation 3, w₀ is an optical spot size 322, or beam waist,diffracting out of the laser 302 by the first surface grating 306, w(z)is a spot size 324 as a function of distance z, e.g., at the secondsurface grating 308, n is an index of refraction of a medium throughwhich the optical beam propagates, and z_(R) is defined on the right inEquation 3.

Assuming the first surface grating 306 of the laser 302 has a size thatis same as the second surface grating 308 in the Si PIC 304 and that abeam output by the laser 302 diverges through the optical isolator 320,the spot size of the beam will grow from w0 322 to w(z) 324. The overlapbetween w(z) 324 and the second surface grating 308 can be used todetermine a coupling loss, assuming a coupling angle is optimal.

An effective distance of the beam through each component (i.e., theoptical isolator 320) between the first surface grating 306 and thesecond surface grating 308 is equal to a physical distance divided by arefractive index of the component or other medium through which the beampropagates. For example, a polarizer of the optical isolator 320 that is40 μm thick with n being equal to 1.5 may have an effective distance of40 μm/1.5, or about 27 μm.

“Working distance” may be a distance in air for which a coupling lossbetween the spot emanating from the laser 302 (or more particularly,from the first surface grating 306 of the laser 302) diverges to a sizeon the second surface grating 308 that leads to a 0.5 dB loss. In someembodiments, thickness of an InP substrate of the laser 302 normalizedto its refractive index of ˜3.2 may be included in the ‘workingdistance’ calculation since the light from the first surface grating 306exits through the InP substrate of the laser 302 before it impinges onthe isolator stack. Given the effective distance of the optical isolator320 and the InP substrate of the laser 302 that is between the firstsurface grating 306 and the second surface grating 308, the spot size,w₀ 322, the first surface grating 306, and the second surface grating308 may be designed to ensure a coupling loss of <0.5 dB. According toat least one embodiment described herein, the working distance betweenthe first surface grating of a surface coupled edge emitting laser andthe second surface grating of a PIC may be at least 100 μm

FIG. 4 illustrates three different optical isolator configurations420A-C, arranged in accordance with at least one embodiment describedherein. The optical isolator configurations 420A-C include a doublestage isolator 420A, a single stage isolator 420B and an isolator withan integrated second polarizer 420C.

The double stage isolator 420A may include a first stack of 2×‘bulk’optical polarizer 424A. As used herein, “bulk” means a micro-opticcomponent not integrated with a Si PIC. The double stage isolator 420Amay also include a Faraday rotator 426A and a second stack of 2×bulkoptical polarizer 424B. The second stack of 2×bulk optical polarizer424B may be designed for roughly 40-50 dB isolation. Additionally, thedouble stage isolator 420A may include a half waveplate 428 to alter orchange a polarization state of a light beam.

The double stage isolator 420A may have an effective thickness ofroughly 600 μm (e.g., 600-800 μm). The double stage isolator 420A may bethe thickest version of the three optical isolators 420A-C discussedherein.

The single stage isolator 420B may include a first bulk polarizer 430A,a Faraday rotator 426B, and a second bulk polarizer 430B. The secondbulk polarizer 430B may be designed for 20 dB isolation. The singlestage isolator 420B may be designed to omit a half waveplate. Forexample, if a second surface grating is designed to receive apolarization state of the second bulk polarizer 430B of the single stageisolator 420B the half waveplate may be omitted. An effective thicknessof the single stage isolator 420B may be roughly 300 μm.

The isolator with integrated second polarizer 420C may be designed toomit a second bulk polarizer, leaving the first bulk polarizer 430C anda Faraday rotator 426C as discrete components of the isolator withintegrated second polarizer 420C. In the isolator with integrated secondpolarizer 420C, a second surface grating on a Si PIC may be designed forone polarization. The second surface grating may serve as a polarizerwhen a second bulk polarizer is omitted. A second integrated polarizerthat includes a Si PIC polarization splitter (not illustrated) may beused as a second polarizer with a 10-20 dB extinction ratio. The secondintegrated polarizer may be located between a fiber-coupled end of a SiPIC and a waveguide connected to the second surface grating that couplesto a first surface grating of a laser.

The polarizers 424A, 430A, and 430C, used to polarize to or from thelaser may typically have a 40-50 dB polarization extinction ratio (PER),defined as ratio of power in a desired polarization divided by power ina rejected polarization of light through the polarizer. Such a 40-50 dBPER polarizer is typically 200 μm thick since it may include twopolarizers with 20-25 dB PER stacked together with a substrate inbetween. It may be desirable to reduce the thickness of such a polarizerto reduce the effective working distance. In these and otherembodiments, the first surface grating of a surface coupled system,which first surface grating may include a InP surface grating, may bedesigned to efficiently diffract light at Transverse Electric (TE)polarization at a desired angle. In this case light of orthogonalpolarization, Transverse Magnetic (TM) is not efficiently coupled andthe first surface grating can function as a polarizer with extinctionratio of 10-20 dB. In such an embodiment, the polarizers 424A, 430A, and430C may be chosen to be thinner (e.g., thinner than 200 μm) and havesmaller polarization extinction ratio of 20-25 dB, which together withthe PER of the first surface grating may provide a similar aggregate40-50 dB PER for a shorter working distance and lower cost. In someapplications when 20 dB PER is sufficient the polarizer 424A, 430A, and430C may be eliminated and the first surface grating may function as a20 dB PER in the isolator stack, providing 20 dB isolation for thelaser.

An example Si PIC polarization splitter is described in U.S. applicationSer. No. 14/938,815 (hereinafter the '815 application), filed on Nov.11, 2015, which is incorporated herein by reference. An advantage ofreplacing the second bulk polarizer with a polarizer integrated in thesecond surface grating may be a relative decrease in effective thicknessbetween a first surface grating and a second surface grating, which mayallow use of a relatively smaller spot size emanating from the firstsurface grating of a laser towards the second surface grating of the SiPIC.

FIG. 5 illustrates a graphical representation 571 of a simulation ofcoupling efficiency (in dB) as a function of a gap distance z (in μm)for multiple spot sizes, arranged in accordance with at least oneembodiment described herein. Curves 573, 575, and 577 respectivelyrepresent coupling efficiency for spot sizes of 10 μm, 14 μm, and 18 μm.The simulation of FIG. 5 considers only the coupling efficiency due todiffraction of the spot as it couples to a second surface grating, e.g.,a receiver grating, in a Si PIC. There may be additional coupling lossdue to finite efficiency of coupling from the receiver grating in the SiPIC to a Si or SiN waveguide in the Si PIC.

For the curve 573, corresponding to the spot size of 10 μm, a couplingefficiency of about −0.5 dB may be achieved at a gap distance (oreffective thickness) of about 160 μm. This gap distance may correspondto an effective thickness of an isolator with integrated secondpolarizer, such as the isolator with integrated second polarizer 420Cdiscussed in relation to FIG. 4. For the curve 575 corresponding to thespot size of 14 μm, a coupling efficiency of about −0.4 dB may beachieved at a gap distance (or effective thickness) of about 300 μm.This gap distance may correspond to an effective thickness of a singlestage isolator, such as the single stage isolator 420B discussed inrelation to FIG. 4. For curve 577 corresponding to the spot size of 18μm, a coupling efficiency of about −0.6 dB may be achieved at a gapdistance (or effective thickness) of about 600 μm. This gap distance maycorrespond to an effective thickness of a double stage isolator, such asthe double stage isolator 420A discussed in relation to FIG. 4.

FIG. 6 illustrates a passive section 632 of an example surface couplededge emitting laser (hereinafter “laser”), arranged in accordance withat least one embodiment described herein. The laser that includes thepassive section 632 of FIG. 6 may be implemented in one or more of thesurface coupled systems discussed herein. The passive section 632 mayinclude a core waveguide 642 optically coupled to a first surfacegrating 606, which may correspond to or include the first surfacegratings discussed elsewhere herein. The passive section 632 may alsoinclude a substrate 634 beneath the core waveguide 642 and the firstsurface grating 606, a dielectric 636 above the core waveguide 642 andthe first surface grating 606, and a top mirror 638 above the dielectric636. The substrate 634 may serve as cladding to the core waveguide 642.In at least one embodiment, the core waveguide 642 may include anInGaAsP waveguide. Some embodiments of a laser may include a top mirrorsuch as the top mirror 638 while other embodiments may omit the topmirror.

The first surface grating 606 may include a periodic structure 640formed at an interface between the core waveguide 642 and the dielectric636. The periodic structure 640 may include a corrugated region thatincludes regions of core waveguide 642 that alternates with regions ofdielectric 636. The regions of core waveguide 642 may have an index ofrefraction that is different than an index of refraction for the regionsof dielectric 636. The periodic regions may be called corrugations orteeth. The periodic structure 640 may expand laterally (e.g., in and outof the page) in the light propagation direction (e.g., from left toright), e.g., in the form of a fan out region.

The dielectric 636 may include SiO2, or SiNx or other suitabledielectric passivation materials. The top mirror 638 may include gold, adielectric stack (e.g., HR coating), or other suitable material and/ormirror. The substrate 634 may include InP or other suitable claddingmaterial.

Although not illustrated in FIG. 6, a laser that includes the passivesection 632 may additionally include an active section that includes again medium and one or more DBR mirrors or DFB reflectors thatcollectively form a laser cavity optically coupled to a core waveguide.

The passive section 632 may be configured to maximize a fraction oflight diffracted by the first surface grating 606 downward through thesubstrate 634. Additionally, the passive section 632 may be configuredto maximize a fraction of diffracted light by the first surface grating606 through the substrate of the laser and out to a second surfacegrating of a Si PIC positioned beneath the laser. A fraction of a lightbeam travelling through the first surface grating 606 may diffract awayfrom the substrate 634 towards the epitaxially grown top surface of thepassive section 632, thereby decreasing a coupling efficiency in adirection towards the substrate 634 and into a Si PIC. As such, the topmirror 638 may be deposited on the dielectric 636 to redirect upwarddiffracted light beams downward through the cladding 634 and into a SiPIC. To ensure the redirected light adds in phase with the lightdiffracted towards the cladding 634 from the first surface grating 606,a thickness of the dielectric 636 may satisfy equation 4:d=m(λ cos(θ))/2n _(dielectric),  Equation 4In equation 4, m is an integer, n is an index of refraction of thedielectric 636, θ is an angle between normal and the propagationdirection of the upward diffracted light, and λ is the wavelength of thelight beam.

FIG. 8A is a graphical representation 879 of a simulation of lightpropagation through a passive section, arranged in accordance with atleast one embodiment described herein. The passive section of FIG. 8Amay be similar to the passive section 632 discussed in relation to FIG.6 but without a top mirror (e.g., the top mirror 638). The passivesection may also have a short length (e.g., a length less than 30 μm).The passive section may include a first surface grating with a gratingperiod of 462.2 nm. Additionally, the first surface grating may have 60periods, and a length of 27.7 μm (60 periods×0.462 m=27.7 μm). As can beseen in the graphical representation 879 of FIG. 8A, a significantportion of light (e.g., 8.3% in the example of FIG. 8A) may be lost outof the passive section due to the absence of the top mirror.

FIG. 8B is a graphical representation 881 of a simulation of lightpropagation through a passive section, arranged in accordance with atleast one embodiment described herein. The passive section of FIG. 8Bmay be similar to the passive section 632 discussed in relation to FIG.6 and may include a top mirror, such as the top mirror 638 discussed inrelation to FIG. 6 above. The passive section may also have a shortlength (e.g., a length less than 30 μm). The passive section may includea first surface grating with a grating period of 462.2 nm. Additionally,the first surface grating may have 60 periods, and a length of 27.7 μm(60 periods×0.462 μm=27.7 μm). As can be seen in the graphicalrepresentation 881 of FIG. 8B, a relatively greater portion of light maybe usable with the light reflecting back down off of the mirror ascompared to FIG. 8A.

It can be seen from a comparison of the simulations of FIGS. 8A and 8Bthat including a top mirror in a passive section may significantlyimprove a coupling efficiency in the downward direction.

FIG. 7 illustrates a passive section 732 of another example surfacecoupled edge emitting laser (hereinafter “laser”), arranged inaccordance with at least one embodiment described herein. The laser thatincludes the passive section 732 may be implemented in one or more ofthe surface coupled systems discussed herein. The passive section 732may include a waveguide core 742 optically coupled to a firstdiffraction grating 706. The first diffraction grating 706 maycorrespond to the other first diffraction gratings and/or first surfacegratings discussed herein. The passive section 732 may also include asubstrate 734 as a bottom cladding beneath the waveguide core 742 andthe first surface grating 706, and a top cladding 736 above thewaveguide core 742 and the first surface grating 706. The top cladding736, waveguide core 742, and bottom cladding 734 together may form awaveguide 744. In this example, light beams may propagate through thewaveguide 744 in a right to left direction. The light beams may travelin the waveguide 744 until the light beam is diffracted in the directionof the bottom cladding 734 by the first diffraction grating 706. Some orall of the diffracted light beam may propagate through and out of thebottom cladding 734.

In some embodiments, the bottom cladding 734 may include InP with abottom cladding index of refraction of about 3.2. Alternatively oradditionally, the top cladding 736 may include a dielectric such as SiO₂with a top cladding index of refraction of about 1.46, or more generallya dielectric with a top cladding index of refraction in a range from1-2. Alternatively or additionally, the waveguide core 742 may includeInGaAsP (sometimes referred to as In_(x)Ga_(1-x)As_(y)P_(1-y) to denotethe ratios of In, Ga, As, and P) with a core index of refraction that ishigher than the bottom cladding index of refraction to guide an opticalmode. Varying the variables x and y in the compositionIn_(x)Ga_(1-x)As_(y)P_(1-y) varies the material bandgap and hencerefractive index and loss.

A wide range of refractive indices for the waveguide core 742 may leadto a guided mode in the waveguide 744. However, in order to achieve highdiffraction efficiency of light that is propagated into the far fieldand can reach a receiver grating in a Si PIC, suitable refractiveindices for the waveguide core 742 may be more limited. In more detail,since the first diffraction grating 706 may include repeated periodicregions of InGaAsP that may have a first index of refraction andperiodic regions of SiO₂ with a second index of refractive, where thefirst index of refraction is higher than the second index of refraction,a resulting effective index of the first diffraction grating 706 may bereduced relative to the waveguide core 742. As a result, if theeffective index of the first diffraction grating 706 becomes similar toor less than an index of refraction for a bottom cladding 734 (e.g., arefractive index of an InP substrate), then light beams diffracting fromthe first diffraction grating 706 may be leaked into substrate modes andmay not diffract into the far field in the desired direction and out ofthe bottom cladding 734 (e.g., an InP substrate) to reach a receivergrating in a Si PIC which is some working distance away.

FIG. 9A illustrates a graphical representation 983 of an example of afar field profile as a function of diffraction angle for a passivesection, such as the passive section 732 discussed in relation to FIG. 7above, arranged in accordance with at least one embodiment describedherein. The passive section may have a core index of refraction of 3.38.As illustrated in FIG. 9A, a large fraction of light may be directedinto leaky modes 985 into the substrate and may not diffract into thefar field. The passive section may have a reduced coupling efficiencyfor coupling light from a laser to a Si PIC and may be impractical for alot of industry uses.

According to at least one embodiment described herein, the core index ofrefraction may be at least 6% higher than the bottom cladding index ofrefraction to provide good efficiency such that the effective index ofthe resulting first diffraction grating is sufficiently higher than thebottom cladding index of refraction (e.g., of the InP substrate) toavoid leakage into the substrate.

Alternatively or additionally, a composition of InGaAsP for a waveguidecore with an index of refraction of 3.40 or higher may generally avoid(or at least reduce compared to the simulation 983 of FIG. 9A) leakageinto the substrate. However, as the index of refraction increases, itmay lead to loss caused by material absorption, so a compromise may bemade to, e.g., balance substrate leakage and loss.

FIG. 9B illustrates various graphical representations of simulations fordiffracted light, arranged in accordance with at least one embodimentdescribed herein. Graph 987 represents a simulation of diffracted poweras a function of a waveguide core index of refraction for a firstdiffraction grating such as the first diffraction grating 706 of FIG. 7.Curve 989 represents total downward diffracted power as a function ofthe waveguide core index of refraction for the first diffractiongrating. Curve 989 accounts for useful diffracted light (e.g., lightactually radiated out of the substrate into the far field) as well aslight that has leaked into the substrate but does not exit thesubstrate. Curve 991 represents far field downward diffracted power as afunction of the waveguide core index of refraction for the firstdiffraction grating. Curve 991 may include only the useful diffractedlight that is actually radiated out of the substrate into the far field.

Graph 993 and graph 995 include far field profiles as a function ofdiffraction angle for two different waveguide core indices ofrefraction. In the simulation represented in graph 993, the waveguidecore may have an index of refraction of 3.38 and much of the light maybe lost to leakage at higher diffraction angles (e.g., angles aboveabout 70 degrees) as denoted at 997. As denoted at 999, the simulationin graph 993 includes a second order radiation mode where light may belost at about a −40 degree diffraction angle. As denoted at 901, thesimulation in graph 993 includes a first order radiation mode at adiffraction angle of about 10 degrees. In the simulation represented ingraph 993, only about 20 percent of total power may be usable andincluded in the first order radiation mode. In comparison, in thesimulation represented in graph 995, the waveguide core index ofrefraction may be 3.42 and there is a single radiation mode denoted at903 at a diffraction angle of about −17 degrees.

The waveguide core index of refraction of an InGaAsP core material maybe adjusted by changing the value(s) of x and/or y in theInxGa1−xAsyP1−y material composition (or other material composition)that makes up a waveguide core. Changing the value(s) of x and/or ychanges the bandgap of In_(x)Ga_(1-x)As_(y)P_(1-y), which in turnchanges its index of refraction. As index of refraction increases withchanges in bandgap, diffraction efficiency decreases due to materialabsorption. Thus, even though an index of refraction of 3.5 may have ahigher far field downward diffracted power than an index of refractionof 3.42, the index of refraction of 3.5 may have too much materialabsorption loss associated with it compared to the index of refractionof 3.42, leading to an overall lower diffraction efficiency. In oneembodiment, a range between 3.4 and 3.44 for the index of refraction ofthe waveguide core 432 may strike a suitable balance between far fielddownward diffracted power and diffraction efficiency in someembodiments. In other embodiments, the index of refraction of thewaveguide core may be less than 3.4 or greater than 3.44 depending onthe particular implementation.

FIG. 10 illustrates a graphical representation 1005 of a simulation ofdiffraction efficiency loss as a function of number N of grating periodsof a first surface grating in a passive section with a top mirror, suchthe passive section 632 of FIG. 6, arranged in accordance with at leastone embodiment described herein. In the simulation of FIG. 10, adiffraction efficiency loss of 0 dB may equal complete (i.e. one hundredpercent) downward diffraction. As can be seen in the graphicalrepresentation 1005 of FIG. 10, as the number N of grating periodsincrease, the diffraction efficiency increases.

FIG. 11 illustrates another passive section 1132 of a laser, arranged inaccordance with at least one embodiment described herein. The laser thatincludes the passive section 1132 of FIG. 11 may be implemented in oneor more of the surface coupled systems discussed herein. The passivesection 1132 may include a top cladding 1136 made of SiO2, SiNx, orother suitable cladding material, a waveguide core 1142 and a firstsurface grating 1106. The first surface grating 1106 may include gratingteeth (i.e., alternating material with different indexes of refraction).The first surface grating 1106 and waveguide core 1142 may be made ofInGaAsP. Additionally, the passive section 1132 may include a bottomcladding 1134 made of a substrate of InP. In at least one embodiment,the waveguide core 1142 may have a height above the bottom cladding 1134of about 350 nm, such as 300-380 nm, 325-375 nm or 350 nm. Alternativelyor additionally, the grating teeth of the first surface grating 1106 mayhave a total height measured from a bottom of the waveguide core 1142 toa top of the grating teeth of the first surface grating 1106 of about650 nm, such as 550-700 nm, 600-680 nm, 650-680 nm, or 673.9 nm.Alternatively or additionally, the grating teeth of the first surfacegrating 1106 may have a height above the waveguide core 1142 measuredfrom a top of the waveguide core 1142 to the top of the grating teeth ofthe first surface grating 1106 of about 300 nm, such as 250-350 nm,310-330 nm, or 323.9 nm.

As illustrated in FIG. 11, the grating teeth of the first surfacegrating 1106 alternate with cladding teeth of the top cladding 1136 andmay therefore have a grating period and/or duty cycle. The gratingperiod may be 525.6 nm, meaning there may be a distance of 525.6 nmbetween a front of each grating tooth and the front of a subsequentgrating tooth. More generally, the grating period may be in a range from500 nm to 600 nm. In an example embodiment, the first surface grating1106 may include 120 grating periods. The duty cycle of the firstsurface grating 1106 may be 0.397, meaning each grating tooth may span39.7% of each grating period where a corresponding top cladding toothoccupies a remainder of each grating period. More generally, the dutycycle may be in a range from 0.3 to 0.5. In an example embodiment, thefirst surface grating 1106 or other first surface gratings describedherein may include one or more of the following parameters: 120 gratingperiods, a grating period of 525.6 nm, a duty cycle of 0.397, a totalgrating tooth height of 673.9 nm, a downward radiation efficiency (DRE)of −0.454 dB, a radiated diffraction angle θ rad of −14.24 degrees, atransmission through the first surface grating of about 3.42%, andreflected power of about −53.6 dB. Here DRE is the useful portion of thelight that is radiated out to the far field and is defined as thefraction of power radiated out of the diffraction grating towards thesubstrate in a small angular window about the radiated diffraction angle□ rad.

FIG. 13 illustrates various graphical representations of the magnitudeof electric field of grating output as a function of location along alength of a first surface grating and a far field profile as a functionof diffraction angle of the first surface grating, arranged inaccordance with at least one embodiment described herein. FIG. 13includes simulations for a first surface grating such as the firstsurface grating 706 of FIG. 7 or the first surface grating 1106 of FIG.11. In more detail, graph 1307 illustrates the magnitude of the electricfield of grating output as a function of location along the length ofthe first surface grating. Graph 1309 illustrates the far field profileas a function of diffraction angle of the first surface grating.

In view of at least FIGS. 7, 9A, 9B, 11, and the associated description,the instant application recognizes various parameters discussed hereinand associated with particular designs for the first surface gratingthat can be included in lasers of one or more surface coupled systemsdescribed herein. Embodiments described herein may include one or moreof these parameters, which will be discussed in connection with FIG. 11above. Although some of the parameters discussed above have beenspecific to InP-based first surface gratings, one or more of thefollowing parameters may be applied to first surface gratings of othermaterial compositions.

First, the total height of the grating teeth may be greater than theheight of the waveguide core. Some diffraction gratings are formed byetching down into a waveguide core to form grating teeth such that theheight of the waveguide core in areas that do not include the gratingteeth is the same as or greater than the total height of the gratingteeth. In comparison, according to some embodiments disclosed herein,the height of the waveguide core in areas that do not include thegrating teeth is less than the total height of the grating teeth.

Second, the grating tooth index of refraction (e.g., the index ofrefraction of the grating teeth that extend upward from the waveguidecore) may be greater than or equal to the core index of refraction(e.g., the index of refraction of the waveguide core). It may be easierto fabricate the grating teeth from the same material composition as thewaveguide core, in which case the grating tooth index of refraction maybe the same as the core index of refraction. In other embodiments, thegrating teeth may be fabricated from a different material compositionthan the waveguide core if the grating tooth index of refraction isgreater than or equal to the core index of refraction.

Third, an effective index of the first diffraction grating may besufficiently higher than the bottom cladding index of refraction (e.g.,the index of refraction of the bottom cladding/substrate) to avoidleakage of a diffracted optical mode into the substrate. For example,the effective index of the first diffraction grating, which depends onat least the core index of refraction and the top cladding index ofrefraction (e.g., the index of refraction of the top cladding) may be atleast 6% higher than the bottom cladding index of refraction.

FIG. 12 illustrates a side cross-sectional view 1202A, a shallow ridgeend-oriented cross-sectional view 1202B, a deep ridge end-orientedcross-sectional view 1202C, and an overhead view 1202D of anotherexample surface coupled edge emitting laser (hereinafter “laser”) 1202,arranged in accordance with at least one embodiment described herein.The laser 1202 may be implemented in one or more of the surface coupledsystems described herein.

The laser 1202 may include an active section 1244 with an active sectionridge structure 1268A and a passive section 1246 with a passive sectionridge structure 1268B. The active section 1244 may include, from bottomto top in the side-cross-sectional view 1202A, a substrate 1248implemented as an n-doped substrate, an InP cladding 1250, a gain layer1252 implemented as a multiple-quantum well (MQW) and core guiding thatmay form a DFB laser, a p-InP layer 1254, an InGaAs or other contactlayer 1256, and a gold contact 1258. The gain layer 1252 within theactive section 1244 may include a MQW sandwiched between upper and lowerwaveguide layers, with a diffraction grating 1260 formed on the upperwaveguide layer.

The passive section 1246 may include, from bottom to top in theside-cross-sectional view, the substrate 1248, the InP cladding 1250, acore waveguide material layer 1264, a first surface grating 1206, and atop mirror 1266 or other HR coating. The core waveguide material layer1264 includes a core waveguide 1268 coupled end to end with the gainlayer 1252, a fan out region 1276 (see overhead view 1202D) coupled endto end with the core waveguide 1268, and a first surface grating 1206formed at the interface between the core waveguide material layer 1264and the top mirror 1266. In some embodiments the first surface grating1206 may be coupled end to end with the fan out region 1276. In someembodiments the first surface grating 1206 may partially overlap the fanout region 1276. In some embodiments the first surface grating 1206 maycompletely overlap the fan out region 1276. The top mirror 1266 mayinclude multiple dielectric layers of alternating indexes of refraction,a gold top mirror or other suitable top mirror or HR coating.

The active section ridge structure 1268A of the laser 1202 may extendthrough the active section 1244. The passive section ridge structure1268B may extend through the passive section 1246. In some embodiments,the active and passive section ridge structures 1268A-B may each have awidth of 2 μm. As illustrated in the two end-oriented cross-sectionalviews 1202B-C the active and passive section ridge structures 1268A-Bmay have different ridge heights. In some embodiments, the activesection ridge structure 1268A may be a shallow ridge with a shorterridge height than the passive section ridge structure 1268B which may bea deep ridge. The active section ridge structure 1268A may extend downto a depth that is above a depth of the gain layer 1252 or to anotherone of the layers of the laser 1202. The passive section ridge structure1268B may extend down to a depth that is below a depth of the gain layer1252 or to another one of the layers of the laser 1202.

The relatively greater ridge height of the passive section ridgestructure 1268B may increase mode confinement. The increased modeconfinement may increase diffraction of output light by the firstsurface grating 1206 and provide a large area mode in a lateraldirection. As described above, the fan out region 1276 and/or the firstsurface grating 1206 may be intended to expand the mode to 8-40 μm or20-40 μm. The expansion of the mode to 8-40 μm or 20-40 μm may beachieved by forming the first surface grating 1206 as a weak (e.g.,small index contrast), long grating in the z direction, where the zdirection is the light propagation direction. Strong confinement in xand y by virtue of the passive section ridge structure 1268B mayincrease diffraction and expand the mode in the x direction. The xdirection refers to the lateral direction (e.g., orthogonal to z andleft to right in the views 1202B-C) and the y direction refers to thevertical direction (e.g., orthogonal to x and z).

Thus, as described with respect to FIG. 12, surface coupled edgeemitting lasers may be implemented as ridge waveguide lasers. In otherembodiments, surface coupled edge emitting lasers as described hereinmay be implemented as buried hetero-structure (BH) lasers. Whetherimplemented as a ridge waveguide laser or a BH laser, some embodimentsof the surface coupled edge emitting lasers described herein may includethe first surface grating “bolted” onto the surface coupled edgeemitting laser to couple light generated by the surface coupled edgeemitting laser out through an upper or lower surface of the surfacecoupled edge emitting laser.

FIG. 14A illustrates a side cross-sectional view of a surface couplededge emitting laser (hereinafter “laser”) 1402, arranged in accordancewith at least one embodiment described herein. The laser 1402 mayinclude or correspond to the other lasers described herein.

In more detail, multiple lasers such as the laser 1402 of FIG. 14A maybe formed in a wafer and formation of a high reflectivity facet mirrorin all of the lasers may be one of the last material additive steps infabrication of the wafer after it has been diced and singulated toexpose the side facets that form the laser cavity. Prior to adding a topmirror to the laser 1402 and the other lasers in the wafer, and/or inembodiments in which a top mirror is omitted altogether, a detector 1470may be positioned above a first surface grating 1406 of the laser 1402.Since the first surface grating 1406 may diffract light both downthrough the substrate and upward, the detector 1470 may be positionedabove the first surface grating 1406 to measure one or more parametersof an optical beam emitted by the laser 1402, a portion of which opticalbeam is diffracted up to the detector 1470 by the first surface grating1406.

A single detector such as the detector 1470 or multiple such detectorsmay be used to measure parameters of optical beams emitted by multiplelasers in the wafer. All of the lasers in the wafer may be measuredsimultaneously, one at a time, in groups of two or more, or in someother manner. High reflectivity facet mirrors may be formed on thelasers after the laser chips have been diced from the wafer andsingulated, in order to expose the facets that form the laser cavity.

FIG. 14B illustrates another example surface coupled system 1400,arranged in accordance with at least one embodiment described herein.The surface coupled system 1400 includes the laser 1402 of FIG. 14A witha top mirror. The surface coupled system 1400 may also include a Si PIC1404 and an optical isolator 1420. Each of the Si PIC 1404 and theoptical isolator 1420 may respectively include or correspond to theother PICs or optical isolators described herein.

In the example of FIG. 14B, the laser 1402 may be a directly modulatedlaser (DML) and may be referred to as “DML laser 1402”. In particular, avoltage or current supplied to the laser 1402 may be modulated so as tomodulate an intensity of an optical beam emitted by the laser 1402. Datamay thereby be encoded in the optical beam. In the example of FIG. 8B,the DML laser 1402 may be p-side down, bonded to a high-speed substrate1488 such as ceramic or Silicon with an appropriate heat sink. In atleast one embodiment, a high-speed driver 1472 and/or a clock and datarecovery (CDR) 1474 chip may be mounted on the high-speed substrate1448. One potential benefit of embodiments discussed herein may be thatsome or all components of the surface coupled system 1400 may be mountedto the high-speed substrate 1448 without needing high accuracy since analignment tolerance may be low. Without a need for high accuracy,mounting of components in accordance with embodiments discussed hereinmay allow for high volume assembly at low cost. For example, the DMLlaser 1402 and/or similar DML lasers may be burned in on sub-mountbefore assembly onto a Si PIC or other package.

FIG. 15 illustrates an overhead view of a core waveguide 1512, a fan outregion 1576, and a first surface grating 1506, arranged in accordancewith at least one embodiment described herein. The core waveguide 1512,the fan out region 1576, and the first surface grating 1506 may beincluded in one or more of the surface coupled edge emitting lasersdescribed herein. Thus, each of the core waveguide 1512, the fan outregion 1576, and the first surface grating 1506 may respectively includeor correspond to the other core waveguides, fan out regions, or firstsurface gratings described herein. The fan out region 1576 may beoptically coupled to the core waveguide 1532 and the first surfacegrating 1506. Although not illustrated, one or both of the fan outregion 1576 and the first surface grating 1506 may include grating linescorresponding to grating teeth as discussed herein. In an example, thefan out region 1576 may have a length R of about 30 μm and a width Y atits widest point of 14.48 μm.

FIG. 16 illustrates a graphical representation 1611 of simulated lightintensity within the core waveguide 1532, the fan out region 1576, andthe first surface grating 1506 of FIG. 15, arranged in accordance withat least one embodiment described herein.

According to some embodiments, a first surface grating such as any ofthe first surface gratings described herein may create a 8-40 μm spotsize or even a 20-40 μm spot size. A size of the spot from the firstsurface grating in an x direction may be determined by diffraction. Thediffraction angle and size of mode in the x direction at a distance Rmay be approximately determined according to equation 5:

$\begin{matrix}{{\theta\text{∼}\frac{\lambda}{\pi\; n\; w_{0}}\mspace{31mu}\Delta\; x\text{∼}\theta\; R},} & {{Equation}\mspace{14mu} 5}\end{matrix}$In equation 5, n may be an effective index of refraction of a fan outregion of a laser that includes the first surface grating. In equation5, w₀ may be a 1/e² Gaussian mode field radius, and λ may be awavelength of light.

For a shallow ridge, the 1/e² Gaussian mode field radius may be roughly1 μm. The effective index of refraction of the fan out region of thelaser may be 3.5 and the wavelength of light may be 1310 nm. In thisembodiment, the diffraction angle θ may be roughly 6.8 degrees. Toobtain a 40 μm spot size, for example, the fan out radius may be roughly335 μm.

For a deep ridge such as the passive section ridge structure 1268B ofFIG. 12, the 1/e2 Gaussian mode field radius may be roughly 0.5 μm. Theeffective index of refraction of the fan out region of the laser may be3.5 and the wavelength of light may be 1310 nm. In this embodiment, thediffraction angle may be roughly 13.6 degrees. To obtain a 40 μm spotsize, the fan out radius may be roughly 167 μm.

FIG. 17 illustrates another example surface coupled system 1700,arranged in accordance with at least one embodiment described herein.The surface coupled system 1700 may include a surface coupled edgeemitting laser (hereinafter “laser”) 1702 that may include or correspondto the other lasers described herein. In an example embodiment, thelaser 1702 may be similar or identical to the laser 1402 of FIGS. 14Aand 14B. The surface coupled system 1700 may also include a Si PIC 1704that may include or correspond to the other PICs described herein. Thesurface coupled system 1700 may include an optical isolator 1720 thatmay include or correspond to the other optical isolators describedherein.

In the surface coupled system 1700, a relatively higher resistance of ap side (labeled “p-InP” in FIG. 17) of the laser 1702 may lead to moreheat generation on the p side of the laser 1702. Accordingly, a heatsink 1778 may be coupled to the p-side of the laser 1702 to providebetter heat sinking. An n doped substrate 1748 of the laser 1702 may bethinned to a few hundred m, to increase a working distance of the laser1702 and the Si PIC 1704 and reduce a required spot size. Alternativelyor additionally, the InP substrate may be semi-insulating.

FIG. 18A illustrates another example of a surface coupled edge emittinglaser (hereinafter “laser”) 1802, arranged in accordance with at leastone embodiment described herein. The laser 1802 may include orcorrespond to one or more other lasers described herein. The laser 1802may be configured to emit light from an epi side or top side of thelaser 1802 as opposed to other lasers that emit light from the bottomside of the lasers. The laser 1802 may include a substrate 1834 and ann-side grating 1882, The n-side grating 1882 may be utilized to providedistributed feedback within a DFB laser section 1813 of the laser 1802.The DFB laser section 1813 includes an active gain material waveguide1816. The n-side grating 1882 is also configured to lie below a passivewaveguide portion 1846 of the laser 1802 and is an example of the firstsurface gratings/transmit gratings described elsewhere herein. Thepassive waveguide portion 1846 includes a passive core waveguidematerial 1864 and is adjacent to the DFB laser section 1813. The n-sidegrating 1882 that is situated under the passive core waveguide 1864 isdesigned so that the passive waveguide portion 1846 of the laser 1802will achieve surface radiation emission. Additionally, the laser 1802may include an HR coating 1890 beneath the n-side grating 1882 under thepassive core waveguide 1864 within a window 1888 formed in the substrate1834.

The portion of the n-side grating 1882 in the passive waveguide portion1846 of the laser 1802 may diffract light both up and down. The upwarddiffracted light may be output to a desired component, such as thedetector 1470 discussed in relation to FIG. 14 above. The HR coating1890 may function analogously to the top mirror 638 described inrelation to FIG. 6 above. In particular, the HR coating 1890 may reflectdownward diffracted light so the downward diffracted and then reflectedlight joins the upward diffracted light and is output to the desiredcomponent. To ensure the reflected light is received in phase with thelight diffracted upward from the first surface grating 1806, thethickness between a bottom of the first surface grating 1806 and the HRcoating 1890 may satisfy equation 4, as described above.

FIGS. 18B and 18C illustrate an alternative embodiment of the laser 1802that is similar to the laser 1802 illustrated in FIG. 18A, but where agrating 1884 for a DFB laser section 1848 and a grating 1806 for apassive laser section 1846 are instead formed on a top side of therespective active waveguide 1816 (or gain section) and passive waveguide1864. The grating 1806 is an example of the first surface gratings ortransmit gratings described elsewhere herein. The laser 1802 asillustrated in FIG. 18B is the same laser 1802 that is in FIG. 18C, butat an earlier stage in a fabrication process thereof.

The laser 1802 includes the active section 1848 with the grating 1884implemented as a DFB grating and may be formed above the gain section1816 of the laser 1802. The gain section 1816 may be formed above acommon etch stop layer 1886 located on a substrate 1834. The common etchstop layer 1886 may be common to both the active section 1844 and thepassive section 1846 of the laser. The passive section 1846 may includethe first surface grating 1806 formed as a p-side grating. The firstsurface grating 1806 may be formed above the passive core waveguidematerial layer 1864 which may be formed above the common etch stop layer1886.

As illustrated in FIG. 18C, one or more layers may be formed above thegain section 1816, such as a p-InP layer 1854 and a p contact 1861layer. Additionally, within the passive section 1846, a window 1888 maybe etched in the substrate 1834. The window 1888 may be formed beneaththe surface grating 1806 that is situated in the passive laser section1846 and up to a bottom surface of the common etch stop layer 1886. A HRcoating 1890 may be applied to the common etch stop layer 1886 in thewindow 1888 beneath the surface grating 1806 to form a bottom mirror.

Light from the DFB section 1848 of the laser 1802 of FIGS. 18B and 18Cwill enter the passive section 1846 where the surface grating 1806 maydiffract light both up and down. The upward diffracted light may beoutput to a desired component, such as the detector 1470 discussed inrelation to FIG. 14 above. The HR coating 1890 may function analogouslyto the top mirror 638 described above in relation to FIG. 6. Inparticular, the HR coating 1890 may reflect light upward so the downwarddiffracted and then reflected light joins the upward diffracted lightand is output to the desired component. To ensure the reflected light isreceived in phase with the light diffracted upward from the firstsurface grating 1806, the thickness between a bottom of the surfacegrating 1806 and the HR coating 1890 may satisfy equation 4, asdescribed above.

A high index contrast surface grating (semiconductor to air orsemiconductor to dielectric) (e.g., the first surface gratings/transmitgratings described herein) may be more difficult to implement on ann-side than a p-side, but a low index contrast surface grating(semiconductor to semiconductor) may be done with equal ease on the nside or the p side of the laser. In some embodiments described herein,the surface grating 1806 formed in the laser 1802 of FIGS. 18B and 18Cmay include a high index contrast surface grating, in which case it maybe easier to form the surface grating 1806 on the p-side.

FIGS. 19A and 19B include an overhead view and a side view of an exampleSi photonic communication module (hereinafter “module”) 1992, arrangedin accordance with at least one embodiment described herein. The module1992 may include multiple transmit channels 1994 and multiple receivechannels 1996.

The module 1992 may include multiple surface coupled edge emittinglasers (hereinafter “lasers”) 1902A-D, each of which may be similar oridentical to one or more of the surface coupled edge emitting lasersdescribed herein. The module 1992 may also include a Si PIC 1904implemented as an optical integrated circuit (OIC), an electricalintegrated circuit (EIC) 1998, a first polymer on glass plug 1901A, anda second polymer on glass plug 1901B.

Each of the four lasers 1902A-D may be configured to emit an opticalbeam at a different one of wavelengths λ1, λ2, λ3, or λ4, respectively.For instance, the laser 1902A may be configured to emit an optical beamat wavelength λ1, the laser 1902B may be configured to emit an opticalbeam at wavelength λ2, the laser 1902D may be configured to emit anoptical beam at wavelength λ3, and the laser 1902D may be configured toemit an optical beam at wavelength λ4. In other embodiments, differentnumbers of lasers on different wavelengths may be implemented in themodule 1992.

The lasers 1902A-D and the OIC 1904 may form a surface coupled system1900, similar or identical to other surface coupled systems describedherein. In more detail, each of the lasers 1902A-D may have a firstsurface grating 1906A-D, such as other first surface gratings describedherein. The OIC 1904 may include multiple second surface gratings1908A-D (e.g., one per laser 1902A-D) such as the other second surfacegratings described herein. An optical isolator 1920 may be positionedbetween the first surface gratings 1906A-D of the lasers 1902A-D and thesecond surface gratings 1908A-D of the OIC 1904. Accordingly, opticalbeams emitted by the lasers 1902A-D may be redirected downward by thefirst surface gratings 1906A-D, through the optical isolator 1920 to thesecond surface gratings 1908A-D, and into one or more Si or SiNwaveguides 1905A-D of the OIC 1904 by the second surface gratings1908A-D.

Thus, in general, each of the optical beams emitted by the lasers1902A-D may be directed into the OIC 1904. In some examples, each of theSi or SiN waveguides 1905A-D may be coupled to a Mach-Zehnder (MZ)interferometer. One or more drivers, electrical traces, and/orelectrodes included in the EIC 1998 may cooperate with the MZinterferometers of the OIC 1904 to form one or more MZ modulators1907A-D. Data may be modulated onto each of the optical beams by the MZmodulators 1907A-D according to any suitable MZ modulation technique. Inother embodiments, the MZ modulators 1907A-D or other external modulatormay be omitted and each of the lasers 1902A-D may include a directlymodulated laser such that each of the lasers 1902A-D may emit an opticalsignal, rather than an unmodulated optical beam, into the OIC 1904.

The optical signals may be received at a multiplexer (hereinafter “mux”)1909, which multiplexes the optical signals onto a common waveguide 1911as a multiplexed signal. In at least one embodiment, the mux 1909 may bea SiN mux and the common waveguide 1911 may be a SiN waveguide. Themultiplexed signal may be adiabatically coupled into a transmit opticalfiber 1913A through the first polymer on glass plug 1901A.

Alternatively or additionally, another multiplexed signal may bereceived from a receive optical fiber 1913B at the second polymer onglass plug 1901B and adiabatically coupled through the second polymer onglass plug 1901B into another common waveguide 1919. A Si PICpolarization splitter 1915 or other suitable polarization splitter maysplit the multiplexed signal according to polarization and may direct aTE polarization mode of the multiplexed signal through a first arm 1917Aand a TM polarization mode of the multiplexed signal through a secondarm 1917B. Each of the TE and TM polarization modes of the multiplexedsignal may be demultiplexed by a corresponding demultiplexer(hereinafter “demux”) 1921A-B to generate multiple receive opticalsignals in each polarization mode. Alternatively or additionally, the TMpolarization mode may be converted to TE polarization before reachingthe demux 1921A. In at least one embodiment, each demux 1921A-B mayinclude a SiN demux.

Each receive optical signal may be received by a corresponding opticalreceiver 1913A-H. In at least one embodiment, each optical receiver1913A-H may include a germanium (Ge) positive-intrinsic-negative (pin)photodiode (hereinafter “Ge pin”). Each optical receiver 1913A-H maygenerate an electrical signal representative of the correspondingreceive optical signal. The corresponding electrical signals may bedirected into the EIC 1998. The EIC 1998 may include multiple electricalsignal adders 1935A-B, only one of which is illustrated in FIG. 19A forsimplicity, the others being denoted by ellipses at 1935B. Eachelectrical signal adder 1935A-B may add an electrical signal thatrepresents an optical receive signal from the first arm 1917A with anelectrical signal that represents a corresponding optical receive signalfrom the second arm 1917B to generate an output electrical signalrepresentative of a corresponding wavelength channel from themultiplexed signal received at the Si PIC polarization splitter 1915.

FIG. 20A illustrates another example Si photonic communication module(hereinafter “module”) 2092, arranged in accordance with at least oneembodiment described herein. The module 2092 may include a surfacecoupled edge emitting laser (hereinafter “laser”) 2002. In at least oneembodiment, the laser 2002 may be implemented as a DML. The module 2092may also include a Si PIC 2004, a first glass waveguide plug 2001A, asecond glass waveguide plug 2001B, an optical receiver 2015, and atrans-impedance amplifier (TIA)/clock and optional data recovery (CDR)integrated circuit (IC) 2031. The first and second glass waveguide plugs2001A and 2001B are examples of interposers as described in the '815application. The first and second glass waveguide plugs 2001A and 2001Bmay each include glass. In other embodiments, polymer waveguide plugsmay be implemented as interposers in the module 2092.

The laser 2002 and the Si PIC 2004 may form a surface coupled system,similar or identical to the other surface coupled systems describedherein. In more detail, the laser 2002 may include a first surfacegrating 2006 similar or identical to the other first surface gratingsdescribed herein and the Si PIC 2004 may include a second surfacegrating 2008 similar or identical to the other second surface gratingsdescribed herein. An optical isolator 2020 may be positioned between thefirst surface grating 2006 and the second surface grating 2008.Accordingly, an optical signal emitted by the laser 2002 may beredirected toward the Si PIC 2004 by the first surface grating 2006,through the optical isolator 2020 to the second surface grating 2008,and into a first waveguide 2005A of the Si PIC 2004 by the secondsurface grating 2008, which first waveguide 2005A may be a SiN waveguideor a Si waveguide. In at least one embodiment, the first waveguide 2005Amay be implemented as a Si waveguide if the second surface grating isSi/SiO₂ or as a SiN waveguide if the second surface grating is SiN.

In at least one embodiment in which the first waveguide 2005A is a Siwaveguide, the optical signal may be adiabatically coupled from thefirst waveguide 2005A into a SiN waveguide of the Si PIC 2004 beforebeing adiabatically coupled from the SiN waveguide to a waveguide of thefirst glass waveguide plug 2001A. Adiabatic coupling is described in the'815 application. In embodiments in which the first waveguide 2005A is aSiN waveguide, the optical signal may be adiabatically coupled directlyfrom the first waveguide 2005A into the waveguide of the first glasswaveguide plug 2001A. The optical signal in the first glass waveguideplug 2001A may be butt-coupled into a transmit optical fiber 2013A.

Another optical signal may be received from a receive optical fiber2013B at the second glass waveguide plug 2001B and adiabatically coupledthrough the second glass waveguide plug 2001B into a second waveguide2005B implemented as a SiN waveguide, which may adiabatically couple thelight into a Si waveguide which, in turn, guides the optical signal toan optical receiver 2015. In at least one embodiment, the opticalreceiver 2015 may include a germanium positive-intrinsic-negative (Gepin) or other suitable optical receiver. The optical signal may bereceived by the optical receiver 2015, which may generate an electricalsignal representative of the optical signal. The electrical signal maybe directed into the TIA/CDR IC 2031 which may amplify, reshape, and/orretime the electrical signal or otherwise process the electrical signal.

In an example implementation, the optical signal received from thereceive optical fiber 2013B may experience a two-stage adiabatictransition from a second waveguide of the second glass waveguide plug2001B to a SiN waveguide to a Si waveguide that carries the opticalsignal to the optical receiver 2015. One or more other optical signalsor optical beams may analogously undergo two-stage adiabatic transitionsfrom an interposer waveguide (such as the second glass waveguide plug2001B) to a SiN waveguide to a Si waveguide, or vice versa. In addition,some embodiments have been described herein as including polymer onglass plugs or glass waveguide plugs, both of which are examples ofinterposers that may be implemented according to some embodiments.Additional details regarding two-stage adiabatic transitions andinterposers that may be suitable for embodiments described herein aredescribed in the '815 application.

FIG. 20B illustrates another example Si photonic communication module(hereinafter “module”) 2093, arranged in accordance with at least oneembodiment described herein. The module 2093 is similar or identical inmany respects to the module 2092 of FIG. 20A and includes, for instance,the laser 2002, the Si PIC 2004, the glass waveguide plugs 2001A, 2001B,the optical receiver 2015, the isolator block 2020, the TIA/CDR IC 2031,and the first and second waveguides 2005A, 2005B. In comparison to FIG.20A, the module 2093 additionally includes an optical spectrum reshaper(OSR) 2033 that together with the directly modulated laser 2002 forms alaser with managed chirp, examples of which are marketed by FINISARCORP. under the name chirp managed laser or CML.

In a laser with managed chirp, the laser 2002 outputs an amplitudemodulated optical signal that also has frequency modulation to the OSR2033. The OSR 2033 may be integrally formed in the Si PIC 2004. The OSR2033 may convert frequency modulation of the optical signal to amplitudemodulation to improve extinction ratio. The OSR 2033 may additionallyintroduce phase correlation between bits of the optical signal toimprove dispersion tolerance. Aspects of an example laser with managedchirp are described in U.S. application Ser. No. 11/968,581, filed onJan. 2, 2008, which is incorporated herein by reference.

FIG. 21 illustrates another example surface coupled system 2100,arranged in accordance with at least one embodiment described herein.The system 2100 may include a surface coupled edge emitting laser(hereinafter “laser”) 2102, an optical isolator 2120, and a Si PIC 2104.Each of the optical isolator 2120 and the Si PIC 2104 may be similar oridentical to the other optical isolators and Si PICs described herein.In at least one embodiment, the laser 2102 may be implemented as ahybrid laser.

The laser 2102 may be formed in an InP wafer bonded to a Si substrate,where InP provides gain and passive mirrors (e.g., Si reflectiongratings) are formed in Si in or on the Si substrate. Light may becoupled from the InP to Si waveguides (not illustrated) within or on theSi substrate by adiabatic coupling or evanescent coupling. Light may becoupled from the InP to the Si waveguide using a small area surfacegrating coupler (not illustrated) which will be discussed in furtherdetail with respect to FIG. 22. Light may also be coupled from the InPto the Si waveguide using edge coupling, which will be discussed infurther detail with respect to FIG. 23.

A small area surface grating coupler may typically be 10-20 μm long andmay be optimized to generate a far-field mode that matches a single modeoptical fiber, e.g., with a 10 μm mode field diameter. As such, opticalbeams from a small area surface grating may have a lens-free workingrange of 50 μm, which may not allow for placement of an optical isolator2120. One potential issue with embodiments through free space with sucha relatively short lens-free working range is that these embodiments mayrequire lenses to accommodate an optical isolator. Such small areasurface grating couplers may also have a relatively large 20-30 nmwavelength bandwidth and may be used to couple light into a Si waveguidefrom a DFB laser using lenses and/or to couple light out of the Siwaveguide into a fiber using lenses.

Embodiments described herein, however, may use one or more LASGs toallow a mode size of 8-40 μm or even 20-40 μm. Accordingly, the laser2102 may additionally include a first surface grating 2106 formedtherein to couple light out of the laser 2102 with a 8-40 μm spot sizeor even a 20-40 μm spot size that emits light vertically out of thelaser 2102. Increasing mode size may increase the lens-free workingdistance to 300-600 μm, which may be large enough to accommodate theoptical isolator 2120 without lenses.

FIG. 22 illustrates various views of an example surface coupled edgeemitting laser (hereinafter “laser”) 2202 that may be implemented in thesurface coupled system of FIG. 21, arranged in accordance with at leastone embodiment described herein. In at least one embodiment, the laser2202 may be implemented as a hybrid laser. FIG. 22 includes a first topview 2202A of the laser 2202, a side view 2202B of the laser 2202, anoptical microscope image 2202C of the laser 2202, a second top view2202D of the laser 2202, and three cross-sectional views 2202E of thelaser 2202.

Light may be coupled between a waveguide 2213 and an InP layer 2233 byone or more small area surface grating couplers 2235A-B (side view 2202Bonly). In at least one embodiment, the waveguide 2213 may be implementedas a Si waveguide. An output coupler 2237 may couple light out of thelaser 2202. In some prior implementations of hybrid lasers such as thelaser 2202, the output coupler 2237 might include a small area surfacegrating coupler. According to embodiments described herein, however, theoutput coupler 2237 may include a LASG, such as the first surfacegratings described herein.

FIG. 23 illustrates various views of another example surface couplededge emitting laser (hereinafter “laser”) 2302 that may be implementedin the surface coupled system 2100 of FIG. 21, arranged in accordancewith at least one embodiment described herein. FIG. 23 includes first,second, and third views of the laser 2302. The first is labeled (a) andincludes a cross-sectional view of the laser 2302 as a hybrid Sievanescent device. The second view is labeled (b) and includes aschematic view of a transition taper of a passive silicon waveguide toan active hybrid section and vice versa of the laser 2302. The thirdview is labeled (c) and includes a scanning electron microscope (SEM)image of a taper of the laser 2302. Although not illustrated in FIG. 23,the laser 2302 may include a LASG, such as any of the first surfacegratings described herein, to couple light out of the Si substrate ofthe laser 2302.

FIG. 24 illustrates another example surface coupled system 2400,arranged in accordance with at least one embodiment described herein.The surface coupled system 2400 may include an input Si waveguide 2413,a Si optical circuit 2439, and first and second LASGs 2408A-B (such asthe second surface gratings described herein). The surface coupledsystem 2400 additionally may include a semiconductor optical amplifier(SOA) 2441 with a third and fourth LASG 2406A-B (such as the firstsurface gratings described herein) at each end of the SOA 2441.

Si photonic components may suffer from high optical loss. In the exampleof FIG. 24, the SOA 2441 may be curved (or not curved) and may becoupled to a Si PIC to provide gain and compensate loss. One opticalisolator (not illustrated) may be positioned between the first LASG2408A and the third LASG 2406A and another optical isolator may bepositioned between the second LASG 2408B and the fourth LASG 2406B.

Thus, FIG. 24 illustrates a system 2400 that includes a surface couplededge emitting optical amplifier implemented as the SOA 2441 with thethird and fourth LASs 2406A-B at opposite ends thereof as well as a PICthat includes the input Si waveguide 2413 and an output Si waveguide(not labeled). The input Si waveguide 2413 is optically coupled to thefirst LASG 2408A. The second LASG 2408B is optically coupled to theoutput Si waveguide.

In the example of FIG. 24, the first LASG 2408A of the PIC may have aworking distance of at least 50 μm. Alternatively or additionally, thesecond LASG 2406B of the SOA 2441 may have a working distance of atleast 50 μm.

FIG. 25A illustrates an example Si PIC 2504 that may be implemented inone or more of the surface coupled systems described herein, arranged inaccordance with at least one embodiment described herein. In general,the Si PIC 2504 may include a Si waveguide layer 2562A, at least one SiNwaveguide layer 2562B, and SiO2 cladding 2534A-B above, below, and/oraround Si and SiN waveguides in the Si and/or SiN waveguide layers2562A-B. A window may be formed in one or more dielectric layers abovethe SiN waveguide layer 2562B to receive an end of an interposer, suchas a polymer or high index glass interposer. In the example illustratedin FIG. 25A, such an interposer includes a polymer waveguide thatincludes a polymer core 2565 and a polymer cladding 2567. In at leastone embodiment, the Si waveguide layer 2562A may include Ge pin diodes,Si modulators, Si mux, and/or other components or devices. Additionaldetails regarding implementations of the Si PIC of FIG. 25A and/or otherSi PICs is described in the '815 application. Although not illustratedin FIG. 25A, the Si PIC 2504 and/or other Si PICs may include a LASG(such as the second surface gratings described herein) to receive lightfrom another LASG (such as the first surface gratings described herein)of a surface coupled edge emitting laser.

FIG. 25B illustrates another example Si PIC 2503 that may be implementedin one or more of the surface coupled systems described herein, arrangedin accordance with at least one embodiment described herein. The Si PIC2503 may be similar in many respects to the Si PIC 2504 of FIG. 25A. Onedifference between the two is that the Si PIC 2503 in FIG. 25B does notinclude a Si waveguide layer. In addition, in FIG. 25B, a LASG 2506(such as the second surface gratings described herein) may be formed ina SiN waveguide layer 2569 of the Si PIC 2503 and may be referred to asa SiN LASG 2506. In an example implementation, the SiN LASG 2506 may beoptically coupled to a laser implemented as a DML so that a Simodulator, often used in a Si PIC to modulate light, is not needed andmay be omitted from the Si PIC 2503. The surface coupling provided bythe SiN LASG 2506 is intended to reduce packaging and assembly cost of aDML based transmitter.

The SiN LASG 2506 of the Si PIC 2503 may have a wider bandwidth than aSi LASG because an effective index of a SiN grating may be smaller thanan effective index of a Si grating. The SiN LASG 2506 may also have alarger coupling angle than a Si LASG. Directionality of diffracted lightmay be improved by adding a mirror (not illustrated) below the SiN LASG2506.

For instance, FIG. 26 illustrates a side cross-sectional view of a SiNLASG 2606 in a PIC 2602 with a mirror 2638, arranged in accordance withat least one embodiment described herein. The mirror 2638 may bepositioned beneath the SiN LASG 2606 a suitable distance to provideconstructive interference between light diffracted upward by the SiNLASG 2606 and light reflected upward by the mirror 2638. The mirror 2638may include a metal reflector layer. In an example embodiment, themirror 2638 may be positioned about 420 nm below the SiN LASG 2606.Forming the mirror 2638 beneath the SiN LASG 2606 may be compatible witha back end of line (BEOL) process of a manufacturer or fab of the Si PIC2602 that includes the SiN LASG 2606.

FIG. 27 illustrates another example surface coupled system 2700,arranged in accordance with at least one embodiment described herein.The surface coupled system 2700 may include a surface coupled edgeemitting laser (hereinafter “laser”) 2702, a first Si PIC 2704A, asecond Si PIC 2704B, an electrical integrated circuit (EIC) 2798, and aninterposer 2701.

The laser 2702 may be electrically coupled through one or more traces2711 in the first Si PIC 2704A to the EIC 2798. The EIC 2798 may includea driver, bias circuitry, and/or other elements to drive the laser 2702to emit an optical beam. The optical beam may be coupled into the firstSi PIC 2704A by a first LASG 2745A formed in the laser 2702 and a secondLASG 2745B formed in the first Si PIC 2704A.

The first Si PIC 2704A may include one or more SiN waveguide layersformed on a Si substrate. In at least one embodiment, a mux 2709 andother components may be formed in at least one of the SiN waveguidelayers. The second LASG 2745B that receives the optical beam from thelaser 2702 may be optically coupled by a first SiN waveguide of thefirst Si PIC 2704A to a third LASG 2745C of the first Si PIC 2704. Theoptical beam may be carried by the first SiN waveguide from the secondLASG 2745B of the first Si PIC 2704A to the third LASG 2745C of thefirst Si PIC 2704A.

The second Si PIC 2704B may include one or more Si waveguide layersformed on a Si substrate, a fourth LASG 2745D, and a fifth LASG 2745E.The second Si PIC 2704B may include one or more MZ interferometers(MZI), pin diodes, waveguide splitters, Si waveguides, or othercomponents or devices formed therein. The optical beam may be coupledfrom the third LASG 2745C of the first Si PIC 2704A to the fourth LASG2745D of the second Si PIC 2704B. The fourth LASG 2745D of the second SiPIC 2704B may be optically coupled to the fifth LASG 2745E of the secondSi PIC 2704B by an MZI or one or more other components or devices of thesecond Si PIC 2704B. The MZI may be electrically coupled, through one ormore traces 2713 in the first Si PIC 2704A, to the EIC 2798, which mayinclude a driver, bias circuitry, or other elements to drive the MZI tomodulate the optical beam received from the laser 2702 through the firstSi PIC 2704A. The modulated optical beam may be output back into thefirst Si PIC 2704A through the fifth LASG 2745E of the second Si PIC2704B and a sixth LASG 2745F of the first Si PIC 2704A.

The mux 2709 may have multiple inputs coupled to multiple SiN waveguidesto receive multiple such modulated optical beams, which modulatedoptical beams may be combined into a common output SiN waveguide. Thecommon output SiN waveguide may be adiabatically coupled to aninterposer waveguide of the interposer, as described in the '815application.

In this and other examples, the first Si PIC 2704A may include only SiNlayers, components, and devices (as opposed to Si layers, components,and devices) to optimize a SiN fab process of the first Si PIC 2704A,whereas the second Si PIC 2704B may include only Si layers, components,and devices (as opposed to SiN layers, components, and devices) tooptimize a Si fab process of the second Si PIC 2704B. For example, thesecond LASG 2745B, third LASG 2745C, and the sixth LASG 2745F in thefirst Si PIC 2704A may include SiN LASGs and may be designed to have anoptimal thickness from a Si substrate of the first Si PIC 2704A toincrease directionality of coupling.

In other embodiments, surface coupled systems may include a laser and asingle Si PIC. The laser may be implemented as a DML and the Si PIC mayinclude one or more SiN layers, components, and devices withoutincluding any Si layers, components, and devices. Such Si PICs may bereferred to as SiN PICs or SiN platforms. The laser may include one ormore of the first surface gratings described herein and the SiN platformmay include one or more of the second surface gratings described herein.The SiN platform can omit Si waveguides or a Si-based MZI or otherSi-based modulator since the laser is directly modulated, in which casethe relatively high loss (e.g., 7-8 dB) of a Si modulator may beavoided.

The SiN Platform in this example may serve as a low cost, lens freepackaging platform allowing lens free, non-critical pick and place.Light may be coupled out of the SiN platform using an adiabaticallycoupled polymer interposer or high index glass interposer. The SiN PICmay essentially include the BEOL of a Si photonic process and mayinclude metal layers as well as a SiN LASG as the second surfacegrating. The SiN platform may include a mux, demux, waveguide splitters,and/or other passive components.

Vertically illuminated pin diodes (InGaAs, Ge/Si, etc.) may be flip chipmounted on the SiN platform and light from SiN waveguides may be coupledto the vertically illuminated pin diodes through a SiN LASG such asdescribed herein. Since high-speed diodes may be about 20 μm tall, andan optical isolator may not be needed between the SiN LASG and the pin,a grating with a 10 μm spot size may be used, although a 20-40 μm spotsize such as may be provided by some LASGs described herein may allowfor large working distance. In this example, incoming light from a fibermay be adiabatically coupled into the SiN platform 2704A through apolymer interposer or high index glass interposer, as described in the'815 application.

In some embodiments, there may be two requirements for formation of aLASG in a photonics platform. Photonics platforms may include SiNplatforms (e.g., one or more SiN waveguide layers on Si substrate), Siplatforms (e.g., one or more Si waveguide layers on Si substrate), andSi/SiN platforms (e.g., one or more Si waveguide layers and one or moreSiN waveguide layers, all on Si substrate). The term “Si PIC” has beenused generically herein to refer to all platforms, unless contextdictates otherwise.

The two requirements may include the following:

-   -   1) Weak, long grating to allow an expansion of the coupled beam        to 8-40 μm or even 20-40 μm        -   a. →Grating pitch is adjusted to maximize    -   2) A waveguide with strong enough confinement to allow for        diffraction of the beam in the lateral direction to achieve a        beam size roughly equal to 8-40 μm or even 20-40 μm

A LASG formed in a Si platform may meet the first condition. However,the mode size in glass waveguides may be largely due to small value ofindex difference between core and cladding, e.g., an index differencemay be roughly 0.006-0.03. A LASG formed in a SiN platform may providehigh enough confinement and an index difference may be roughly 0.5 sothat a waveguide mode with a width roughly equal to 1 μm may beachievable, requiring a fan out length of roughly 250 μm. The choicebetween a Si platform and a SiN platform may depend on directionality(ratio of power diffracted up vs. down), which may also be a function ofthe substrate and a SiO₂ box layer.

FIG. 28 illustrates an example focusing surface grating 2906 that may beimplemented in one or both of the first and second surface gratings orother LASGs described herein, arranged in accordance with at least oneembodiment described herein.

FIG. 29 depicts an example concept to increase directionality of asurface grating that may be implemented in one or both of the first andsecond surface gratings described herein, arranged in accordance with atleast one embodiment described herein. In the example of FIG. 29, onesurface grating 3001 may be formed over another surface grating 3003,where one is shifted relative to the other. The two stacked surfacegratings may be implemented together as one of the first surfacegratings to diffract light out of a laser or as one of the secondsurface gratings to diffract light into a Si PIC as described herein.

FIG. 30A illustrates another example surface coupled system 3100,arranged in accordance with at least one embodiment described herein.The system 3100 may include a surface coupled edge emitting laser(hereinafter “laser”) 3102. The laser 3102 may include or correspond toany of the other lasers described herein. The laser 3102 may beimplemented as an InP laser. The laser 3102 may be in a same chip withan InP MZ modulator 3107, where the laser 3102 and InP MZ modulator 3107may be separated by an angled isolation trench 3157. The laser 3102 mayinclude a first LASG 3145A to couple light out of the laser 3102. TheInP MZ modulator 3107 may include a second LASG 3145B to couple lightinto the InP MZ modulator 3107.

The surface coupled system 3100 may also include a bridge 3159. Thebridge 3159 may include a third LASG 3145C to couple light into thebridge 3159 from the first LASG 3145A of the laser 3102 and a fourthLASG 3145D to couple light out of the bridge 3159 into the second LASG3145B of the InP MZ modulator 3107.

The surface coupled system 3100 may additionally include first andsecond optical isolators 3120A-B. The first optical isolator 3120A mayinclude a first polarizer 3124A and a first Faraday rotator 3126A. Thefirst polarizer 3124A and the first Faraday rotator 3126A may be locatedbetween the first LASG 3145A of the laser 3102 and the third LASG 3145Cof the bridge 3159. The second optical isolator 3120B may include asecond polarizer 3124B and a second Faraday rotator 3126B. The secondpolarizer 3124B and the second Faraday rotator 3126B may be locatedbetween the fourth LASG 3145D of the bridge 3159 and the second LASG3145B of the InP MZ modulator 3107.

Within an InP substrate 3134A-C of the laser 3702 and InP MZ modulator3107, multiple windows 3188A-B may be etched beneath a correspondingLASG 3145A-B. An HR coating 3190A-B may be applied beneath thecorresponding LASG 3145A-B to form bottom mirrors. Similarly, within anInP substrate 3134D of the bridge 3159, multiple windows 3188C-D may beetched above the corresponding LASG 3145C-D. An HR coating 3190C-D maybe applied above the corresponding LASG 3145C-D to form top mirrors. Insome embodiments, the bridge 3159 may be formed in a same wafer as thelaser 3102 and the InP MZ modulator 3107, in which case the bridge 3159may be flipped upside down after fabrication.

In operation, the laser 3102 may emit an optical beam, which may includea continuous wave (CW) beam in some embodiments. The optical beam may becoupled from the laser 3102 through the first optical isolator 3120A tothe bridge 3159 by the first LASG 3145A of the laser 3102 and the thirdLASG 3145C of the bridge 3159. The third LASG 3145C of the bridge 3159may be optically coupled to the fourth LASG 3145D of the bridge 3159 andmay redirect the optical beam to the fourth LASG 3145D of the bridge3159. The fourth LASG 3145D of the bridge 3159 may couple the opticalbeam out of the bridge 3159 through the second optical isolator 3120Band the second LASG 3145B of the InP MZ modulator 3107 into the InP MZmodulator 3107, where the optical beam may be modulated to form anoptical signal with information encoded thereon.

In one embodiment, it may be useful to isolate the laser 3102 from timemodulated reflections within the InP MZ modulator 3107. In this example,two stage isolation may be achieved by hybrid integration. The bridge3159 may be cleaved out of a same wafer as the laser 3102 and the InP MZmodulator 3107 such that the bridge 3159 may have a match in terms oflayer thicknesses, compositions, and LASG grating depth with the laser3102 and the InP MZ modulator 3107. Having matching terms of layerthicknesses, compositions, and LASG grating depth may result in optimalLASG-to-LASG insertion loss performance. The laser 3102, InP MZmodulator 3107, and bridge 3159 may be delivered together on the samewafer for simplified inventory management.

FIG. 30A illustrates an example implementation of the surface coupledsystem 3100 of FIG. 30A, arranged in accordance with at least oneembodiment described herein.

FIG. 31 illustrates another example surface coupled system 3200,arranged in accordance with at least one embodiment described herein.The system 3200 is similar to the system 3100 of FIGS. 30A and 30Bexcept that a laser 3202 and an InP MZ modulator 3207 may be completelyseparated to take advantage of placement tolerance and reduce total InPchip cost.

FIG. 32 illustrates another example surface coupled system 3300,arranged in accordance with at least one embodiment described herein.The system 3300 may include a bridge 3359 that includes a semiconductoroptical amplifier (SOA) 3341 between a first LASG 3345A and a secondLASG 3345B. The first and second LASGs 3345A-B may include P contacts3361A-B and N contacts 3363 on a back side of the 3359. The bridge 3359may also include multiple p-dopant diffused feedthroughs 3365A-B. In anyof the example embodiments illustrated in FIGS. 30A, 30B, 31, and 32,the waveguide section within the bridge that is situated between thethird and fourth LASG 3145C and 3145D can effect a change in directionwithin the bridge, namely that the waveguide does not have to remainrectilinear between the third LASG 3145C and fourth LASG 3145D. A changein direction of this waveguide section could be included to facilitatethe assembly of the subcomponents (3102 and 3159) themselves, and/or toimprove the optical isolation performance of the completed device.

The concept of LASGs may be extended to other InP devices besideslasers. For example, electro-absorption modulators (EAMs), InP MZmodulators and/or other InP devices. In these and other embodiments,LASGs included in the InP devices and/or Si PICs may be designed to havea 8-40 μm spot size or even a 20-40 μm spot size to allow a highalignment tolerance and lens free assembly, to thereby reduce costs.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A system comprising: a surface coupled edgeemitting laser comprising: a core waveguide; a fan out region opticallycoupled to the core waveguide in a same layer of the surface couplededge emitting laser as the core waveguide; a first surface gratingformed in the fan out region, wherein the first surface gratingdiffracts light emitted by the surface coupled edge emitting laserupward and downward; one or more lower layers beneath the corewaveguide, wherein the one or more lower layers are substantiallytransparent to light emitted by the surface coupled edge emitting laser;a top mirror positioned above the first surface grating, wherein the topmirror reflects upward-diffracted light downward; a dielectricpositioned above the core waveguide and the first surface grating andbelow the top mirror, wherein the dielectric contains a thickness toallow the reflected upward-diffracted light to add in phase withdownward-diffracted light; an active section that includes a distributedfeedback (DFB) laser, wherein the DFB laser is positioned above the oneor more lower layers and is optically coupled end to end with the corewaveguide; a passive section that includes a core region and the fan outregion, wherein the passive section is optically coupled end to end withthe active section and includes the top mirror; and a photonicintegrated circuit (PIC) comprising an optical waveguide and a secondsurface grating formed in an upper layer of the PIC, wherein the secondsurface grating is in optical alignment with the first surface grating.2. The system of claim 1, further comprising an optical isolatorpositioned between the first surface grating and the second surfacegrating, wherein: the first surface grating is configured to redirectlight received from the core waveguide downward toward the secondsurface grating; the second surface grating is positioned to receive thelight from the first surface grating after the light passes through theoptical isolator; and the second surface grating is configured toredirect light received from the first surface grating into the opticalwaveguide.
 3. The system of claim 1, wherein each of the first surfacegrating and the second surface grating comprises a long surface gratingto produce a large diffracted spot.
 4. The system of claim 1, furthercomprising an optical isolator positioned between the first surfacegrating and the second surface grating, wherein: the optical isolatorhas a physical thickness in a range of 300-800 micrometers (μm); and theoptical isolator comprises a garnet with an input polarizer coupled toan upper surface of the garnet and an output polarizer coupled to alower surface of the garnet.
 5. The system of claim 4, wherein a surfacearea of each of an upper surface of the optical isolator and a lowersurface of the optical isolator is less than 200 μm².
 6. A systemcomprising: a surface coupled edge emitting laser comprising: a corewaveguide; a fan out region optically coupled to the core waveguide in asame layer of the surface coupled edge emitting laser as the corewaveguide; a first surface grating formed in the fan out region, whereinthe first surface grating diffracts light emitted by the surface couplededge emitting laser; one or more lower layers beneath the corewaveguide, wherein the one or more lower layers are substantiallytransparent to light emitted by the surface coupled edge emitting laser;an active section that includes a distributed feedback (DFB) laser,wherein the DFB laser is positioned above the one or more lower layersand is optically coupled end to end with the core waveguide; a passivesection that includes a core region and the fan out region, wherein thepassive section is optically coupled end to end with the active sectionand includes a top mirror; and a photonic integrated circuit (PIC)comprising an optical waveguide and a second surface grating formed inan upper layer of the PIC, wherein the second surface grating is inoptical alignment with the first surface grating; wherein the surfacecoupled edge emitting laser further comprises a ridge structure formedin the active section and the passive section, wherein the ridgestructure comprises a shallow ridge that extends downward to a depth inthe active section above a depth of a multiple quantum well layer of theactive section and a deep ridge that extends downward to a depth in thepassive section below the depth of the multiple quantum well layer. 7.The system of claim 1, wherein the surface coupled edge emitting lasercomprises a directly modulated laser (DML).
 8. The system of claim 7,further comprising an electrical integrated circuit (EIC) mechanicallyand electrically coupled to the PIC, wherein electrical signals arecarried from the EIC to the surface-coupled edge emitting laser throughhigh-speed lines in the PIC.
 9. The system of claim 8, wherein the PICfurther comprises an optical multiplexer.
 10. The system of claim 1,wherein the first surface grating comprises a focusing grating.
 11. Thesystem of claim 7, further comprising a high speed substrate, whereinthe surface coupled edge emitting laser is bonded to the high speedsubstrate p-side down.
 12. The system of claim 11, further comprising atleast one of a high-speed driver or a clock and data recovery (CDR)circuit mounted to the high speed substrate and communicatively coupledto the surface coupled edge emitting laser.
 13. The system of claim 7,wherein the optical waveguide of the PIC includes a silicon nitride(SiN) waveguide, the system further comprising an interposer thatincludes an interposer waveguide adiabatically coupled at one end of theinterposer waveguide to the SiN waveguide and butt-coupled at anopposite end of the interposer waveguide to an optical fiber.
 14. Thesystem of claim 7, wherein the optical waveguide of the PIC includes asilicon (Si) waveguide and the PIC further includes a silicon nitride(SiN) waveguide adiabatically coupled to the Si waveguide, the systemfurther comprising an interposer that includes an interposer waveguideadiabatically coupled at one end of the interposer waveguide to the SiNwaveguide and butt-coupled at an opposite end of the interposerwaveguide to an optical fiber.
 15. The system of claim 1, wherein thesecond surface grating comprises a silicon nitride (SiN) diffractiongrating.
 16. The system of claim 1, wherein the surface coupled edgeemitting laser further comprises: a substrate above which the corewaveguide, the fan out region, and the first surface grating are formed,the substrate defining a window formed therein beneath the first surfacegrating; and a high reflectivity coating formed in the window beneaththe first surface grating.
 17. The system of claim 1, wherein a workingdistance between the first surface grating and the second surfacegrating is at least 100 micrometers.
 18. The system of claim 1, whereinthe first surface grating is configured to generate a spot size in arange from 8 micrometers (μm) to 40 μm.
 19. The system of claim 1,wherein the surface coupled edge emitting laser comprises indiumphosphide.
 20. The system of claim 1, wherein the surface coupled edgeemitting laser comprises a hybrid laser.
 21. The system of claim 17,further comprising a first optical isolator positioned in a firstoptical path between the first surface grating and a third surfacegrating and without any lenses in the first optical path and a secondoptical isolator positioned in a second optical path between the secondsurface grating and a fourth surface grating and without any lenses inthe second optical path.
 22. A system comprising: a surface coupled edgeemitting laser comprising a first waveguide and a first diffractiongrating optically coupled to the first waveguide; and a photonicintegrated circuit (PIC) comprising a second waveguide and a seconddiffraction grating optically coupled to the second waveguide, wherein:the first waveguide of the surface coupled edge emitting laser comprisesa core with a core index of refraction, a top cladding with a topcladding index of refraction, and a substrate as a bottom cladding witha bottom cladding index of refraction; the first diffraction gratingcomprises grating teeth formed on the core of the first waveguide, thegrating teeth extending upward from a top of the core of the firstwaveguide, the grating teeth each having a total height, a height abovethe top of the core of the first waveguide, a period, and a duty cycle;and the core index of refraction is greater than a first threshold valueso that an effective index of the first diffraction grating issufficiently higher than the bottom cladding index to avoid leakage of adiffracted optical mode into the substrate.
 23. The system of claim 22,further comprising an optical isolator positioned in an optical pathbetween the first diffraction grating and the second diffraction gratingand without any lenses in the optical path between the first diffractiongrating and the second diffraction grating.
 24. The system of claim 22,wherein the core index of refraction is in a range from 3.4 to 3.44. 25.The system of claim 22, wherein: a height of the core is 300-380nanometers (nm); the total height of the grating teeth is 600-680 nm;the core index of refraction is in a range from 3.4 to 3.44; the bottomcladding comprises indium phosphide; and the top cladding index ofrefraction is in a range from 1 to
 2. 26. The system of claim 25,wherein the period of the first diffraction grating is in a range from500-600 nm and the duty cycle of the first diffraction grating is in arange from 0.3-0.5.
 27. The system of claim 22, wherein the effectiveindex of the first diffraction grating depends on at least the coreindex of refraction and the top cladding index of refraction and is atleast 6% higher than the bottom cladding index of refraction.
 28. Thesystem of claim 22, wherein the top cladding includes top cladding teeththat alternate with the grating teeth in the first diffraction grating,the top cladding teeth having the top cladding index of refraction andthe grating teeth have a grating tooth index of refraction that isgreater than or equal to the core index of refraction.
 29. The system ofclaim 28, wherein the grating teeth have a same material composition asthe core such that the core index of refraction and the grating toothindex of refraction are the same.
 30. A system comprising: a surfacecoupled edge emitting semiconductor optical amplifier (SOA) comprising acore waveguide, a first fan out region optically coupled to a first endof the core waveguide in a same layer of the surface coupled edgeemitting SOA as the core waveguide, a first surface grating formed inthe first fan out region, a second fan out region optically coupled to asecond end of the core waveguide in the same layer of the surfacecoupled edge emitting SOA as the core waveguide, and a second surfacegrating formed in the second fan out region; and a photonic integratedcircuit (PIC) comprising a first optical waveguide, a second opticalwaveguide, a third surface grating formed in an upper layer of the PICand optically coupled to the first optical waveguide, and a fourthsurface grating formed in the upper layer of the PIC and opticallycoupled to the second optical waveguide, wherein the third surfacegrating is in optical alignment with the first surface grating of thesurface coupled edge emitting SOA and the fourth surface grating is inoptical alignment with the second surface grating of the surface couplededge emitting SOA.
 31. The system of claim 30, wherein a workingdistance between the first surface grating and the third surface gratingand between the second surface grating and the fourth surface grating isat least 50 micrometers.
 32. The system of claim 30, wherein each of thefirst surface grating and the third surface grating is configured togenerate a spot size in a range from 8 micrometers (μm) to 40 μm. 33.The system of claim 6, wherein the deep ridge increases modeconfinement, the increased mode confinement increasing the diffractionof light by the first surface grating.
 34. The system of claim 6,wherein the shallow ridge extends through the entirety of the activesection and the deep ridge extends through the entirety of the passivesection.